As in QVarLengthArray (c34242c679), this will
probably bite someone someday, so fix it before there's a bug report.
Use ctor delegation to keep code size increase small.
There's also the benefit that default-constructing a QNetworkRequest now
no longer creates an expensive QUrl object just to destroy it unused again.
Add an auto-test.
Change-Id: I5ceb5402ca3946048d695244d1b1c36564a1e80a
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
a7383b4b6d