Fix setRequestHeader to only append to headers added by user.
Fix setRequestHeader to only append to headers added by user.

This causes no behavior change. The reason for this patch is due to a planned
change in QNetworkRequest that causes default headers to be added on creation.

Change-Id: Ic233a12eb0aa35475300d65738ee26872349518b
Reviewed-by: Shane Kearns <47c14d25d405178da551f2d3fdd4e2e2937090fc@accenture.com>

mgrunditz/qtdeclarative-2d,matthewvogt/qtdeclarative,matthewvogt/qtdeclarative,mgrunditz/qtdeclarative-2d,corngood/qtdeclarative,corngood/qtdeclarative,qmlc/qtdeclarative,corngood/qtdeclarative,qmlc/qtdeclarative,qmlc/qtdeclarative,mgrunditz/qtdeclarative-2d,matthewvogt/qtdeclarative,qmlc/qtdeclarative,mgrunditz/qtdeclarative-2d,corngood/qtdeclarative,corngood/qtdeclarative,corngood/qtdeclarative,qmlc/qtdeclarative,matthewvogt/qtdeclarative,qmlc/qtdeclarative,matthewvogt/qtdeclarative,matthewvogt/qtdeclarative,qmlc/qtdeclarative,qmlc/qtdeclarative,mgrunditz/qtdeclarative-2d,matthewvogt/qtdeclarative,mgrunditz/qtdeclarative-2d,corngood/qtdeclarative,matthewvogt/qtdeclarative,mgrunditz/qtdeclarative-2d