[REG 5.15] Custom scheme responses are cut off after 64k

In the Qt 5.15 Alpha, custom URL scheme handler responses are cut off after 64k of data.

This can be observed with the webui example, by replacing about.html with this modified file (the only change is 80 added "lorem ipsum" paragraphs).

When compiling against 5.14 and launching the application, the entire text and the "quit" button is shown. With 5.15, the shown page is cut off in the middle of the text. When inspecting the network reply via QTWEBENGINE_REMOTE_DEBUGGING, it becomes apparent that the reply got cut off after ~64k bytes.