LibWeb: Implement '5.4. Request class' from the Fetch API :^)

Author: https://github.com/linusg
Commit: 9fb672e981
Pull-request: https://github.com/SerenityOS/serenity/pull/15359
Reviewed-by: https://github.com/alimpfard
Reviewed-by: https://github.com/davidot ✅
Reviewed-by: https://github.com/kennethmyhra

Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Headers.h

@@ -31,6 +31,7 @@ class HeaderList final
     , Vector<Header> {
 public:
     using Vector::begin;
+    using Vector::clear;
     using Vector::end;
 
     [[nodiscard]] bool contains(ReadonlyBytes) const;

Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Requests.cpp

@@ -36,7 +36,8 @@ void Request::set_url(AK::URL url)
     // Sometimes setting the URL and URL list are done as two distinct steps in the spec,
     // but since we know the URL is always the URL list's first item and doesn't change later
     // on, we can combine them.
-    VERIFY(m_url_list.is_empty());
+    if (!m_url_list.is_empty())
+        m_url_list.clear();
     m_url_list.append(move(url));
 }