LibWeb/Fetch: Use MimeType in DataURL

Author: https://github.com/jamierocks
Commit: 295c4ef51a
Pull-request: https://github.com/SerenityOS/serenity/pull/24525

Userland/Libraries/LibWeb/Fetch/Fetching/Fetching.cpp

@@ -851,8 +851,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<PendingResponse>> scheme_fetch(JS::Realm& r
             return PendingResponse::create(vm, request, Infrastructure::Response::network_error(vm, "Failed to process 'data:' URL"sv));
 
         // 3. Let mimeType be dataURLStruct’s MIME type, serialized.
-        //    FIXME: Serialize MIME type.
-        auto const& mime_type = data_url_struct.value().mime_type;
+        auto const& mime_type = MUST(data_url_struct.value().mime_type.serialized());
 
         // 4. Return a new response whose status message is `OK`, header list is « (`Content-Type`, mimeType) », and
         //    body is dataURLStruct’s body as a body.