Fix memory corruption in XHR implementation

Now that the member data of Object is also on the GC heap, it is even more
important to call the base implementation of markObjects when re-implementing
it ;-)

Fixes also three XHR auto tests (text, cdata and stateChangeCallingContext)

Change-Id: Ifd033cac64a8cf27a9186d8cd9eb94369d8a60a5
Reviewed-by: Michael Brasser <michael.brasser@live.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Fix XHR auto tests always failing

The test (rightly so) expects that even for a 400 response, the responseBody
must be preserved - as per commit 0949071f13e7bcbc16a0f07f496e0b6a23b04edd.
However before commit fd5dd2712656cbc674c8360754394e41dd82e40c in qtbase, 400
was mapped to QNetworkReply::UnknownContentError, which is "handled" by
QQmlXMLHttpRequest::error in the sense that the body is preserved. After the
qtbase behavioral change, 400 now maps to
QNetworkReply::ProtocolInvalidOperationError, which was _not_ handled, the body
was cleared and the test failed.

The revdep of qtdeclarative for qtbase should've caught the behavioral change
in qtbase, but unfortunately the auto-test was disabled (marked insignificant).

This patch adjusts the xhr implementation to the behavioral change in qtbase
and also re-activates the xhr auto-test (we really should be testing our xhr
implementation in the CI system...).

Task-number: QTBUG-38180
Change-Id: I7655fd9037752ad644a98d01257bf5f81e7daa15
Reviewed-by: Mandeep Sandhu <mandeepsandhu.chd@gmail.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Peter Hartmann <phartmann@blackberry.com>