Support DELETE requests with a body

While non-standard, other browsers allow DELETE requests to send a body
in the HTTP request. Since there is no specific API for delete requests
with a body in QNetworkAccessManager, we use custom HTTP request for
this.

Task-number: QTBUG-42456
Change-Id: I01a055ea173b5a65086f97d0c6ef14a9e01141b3
Reviewed-by: Milian Wolff <milian.wolff@kdab.com>
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>