Http[s]Download: Make the constructor's initialization DRY

Problem:
- `HttpDownload()` and `HttpsDownload()` implementations are the same
  except for types and certificates.

Solution:
- Follow the "Don't Repeat Yourself" mantra and de-duplicate the code
  using templates.

Userland/Services/ProtocolServer/Download.h

@@ -55,9 +55,6 @@ public:
     void set_download_fd(int fd) { m_download_fd = fd; }
     int download_fd() const { return m_download_fd; }
 
-protected:
-    explicit Download(ClientConnection&, NonnullOwnPtr<OutputFileStream>&&);
-
     void did_finish(bool success);
     void did_progress(Optional<u32> total_size, u32 downloaded_size);
     void set_status_code(u32 status_code) { m_status_code = status_code; }
@@ -66,6 +63,9 @@ protected:
     void set_downloaded_size(size_t size) { m_downloaded_size = size; }
     const OutputFileStream& output_stream() const { return *m_output_stream; }
 
+protected:
+    explicit Download(ClientConnection&, NonnullOwnPtr<OutputFileStream>&&);
+
 private:
     ClientConnection& m_client;
     i32 m_id { 0 };

Userland/Services/ProtocolServer/HttpCommon.h

@@ -28,14 +28,50 @@
 
 #include <AK/ByteBuffer.h>
 #include <AK/HashMap.h>
+#include <AK/NonnullOwnPtr.h>
+#include <AK/Optional.h>
 #include <AK/OwnPtr.h>
 #include <AK/String.h>
+#include <AK/Types.h>
 #include <LibHTTP/HttpRequest.h>
 #include <ProtocolServer/ClientConnection.h>
 #include <ProtocolServer/Download.h>
 
 namespace ProtocolServer::Detail {
 
+template<typename TSelf, typename TJob>
+void init(TSelf* self, TJob job)
+{
+    job->on_headers_received = [&](auto& headers, auto response_code) {
+        if (response_code.has_value())
+            self->set_status_code(response_code.value());
+        self->set_response_headers(headers);
+    };
+
+    job->on_finish = [&](bool success) {
+        if (auto* response = job->response()) {
+            self->set_status_code(response->code());
+            self->set_response_headers(response->headers());
+            self->set_downloaded_size(self->output_stream().size());
+        }
+
+        // if we didn't know the total size, pretend that the download finished successfully
+        // and set the total size to the downloaded size
+        if (!self->total_size().has_value())
+            self->did_progress(self->downloaded_size(), self->downloaded_size());
+
+        self->did_finish(success);
+    };
+    job->on_progress = [&](Optional<u32> total, u32 current) {
+        self->did_progress(total, current);
+    };
+    if constexpr (requires { job->on_certificate_requested; }) {
+        job->on_certificate_requested = [&](auto&) {
+            self->did_request_certificates();
+        };
+    }
+}
+
 template<typename TBadgedProtocol, typename TPipeResult>
 OwnPtr<Download> start_download(TBadgedProtocol&& protocol, ClientConnection& client, const String& method, const URL& url, const HashMap<String, String>& headers, ReadonlyBytes body, TPipeResult&& pipe_result)
 {

Userland/Services/ProtocolServer/HttpDownload.cpp

@@ -25,7 +25,7 @@
  */
 
 #include <LibHTTP/HttpJob.h>
-#include <LibHTTP/HttpResponse.h>
+#include <ProtocolServer/HttpCommon.h>
 #include <ProtocolServer/HttpDownload.h>
 #include <ProtocolServer/HttpProtocol.h>
 
@@ -35,29 +35,7 @@ HttpDownload::HttpDownload(ClientConnection& client, NonnullRefPtr<HTTP::HttpJob
     : Download(client, move(output_stream))
     , m_job(job)
 {
-    m_job->on_headers_received = [this](auto& headers, auto response_code) {
-        if (response_code.has_value())
-            set_status_code(response_code.value());
-        set_response_headers(headers);
-    };
-
-    m_job->on_finish = [this](bool success) {
-        if (auto* response = m_job->response()) {
-            set_status_code(response->code());
-            set_response_headers(response->headers());
-            set_downloaded_size(this->output_stream().size());
-        }
-
-        // if we didn't know the total size, pretend that the download finished successfully
-        // and set the total size to the downloaded size
-        if (!total_size().has_value())
-            did_progress(downloaded_size(), downloaded_size());
-
-        did_finish(success);
-    };
-    m_job->on_progress = [this](Optional<u32> total, u32 current) {
-        did_progress(total, current);
-    };
+    Detail::init(this, job);
 }
 
 HttpDownload::~HttpDownload()

Userland/Services/ProtocolServer/HttpsDownload.cpp

@@ -24,8 +24,8 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#include <LibHTTP/HttpResponse.h>
 #include <LibHTTP/HttpsJob.h>
+#include <ProtocolServer/HttpCommon.h>
 #include <ProtocolServer/HttpsDownload.h>
 #include <ProtocolServer/HttpsProtocol.h>
 
@@ -35,32 +35,7 @@ HttpsDownload::HttpsDownload(ClientConnection& client, NonnullRefPtr<HTTP::Https
     : Download(client, move(output_stream))
     , m_job(job)
 {
-    m_job->on_headers_received = [this](auto& headers, auto response_code) {
-        if (response_code.has_value())
-            set_status_code(response_code.value());
-        set_response_headers(headers);
-    };
-
-    m_job->on_finish = [this](bool success) {
-        if (auto* response = m_job->response()) {
-            set_status_code(response->code());
-            set_response_headers(response->headers());
-            set_downloaded_size(this->output_stream().size());
-        }
-
-        // if we didn't know the total size, pretend that the download finished successfully
-        // and set the total size to the downloaded size
-        if (!total_size().has_value())
-            did_progress(downloaded_size(), downloaded_size());
-
-        did_finish(success);
-    };
-    m_job->on_progress = [this](Optional<u32> total, u32 current) {
-        did_progress(total, current);
-    };
-    m_job->on_certificate_requested = [this](auto&) {
-        did_request_certificates();
-    };
+    Detail::init(this, job);
 }
 
 void HttpsDownload::set_certificate(String certificate, String key)