Change default Content-Type for http POST to match 4.7.4

Change default Content-Type for http POST to match 4.7.4

Qt 4.7.3 sent no Content-Type header by default.
This was fixed independently on 4.8.0 and 4.7.4 branches, with
different defaults.
Since this is often used for web service logins, the 4.7.4
default of x-www-form-urlencoded seems more likely to work.

The warning message is left in place, since not specifying the
content type is still an application bug.

Task-number: QTBUG-23350
Change-Id: I30bf50fd216ee9894d0168e904cea1ed4251ec68
Reviewed-by: Martin Petersson <Martin.Petersson@nokia.com>