Make TestHTTPServer header comparison order independent.

The order of headers in an HTTP request is irrelevant and program
behavior should not depend on it.

Change-Id: I77e1b6265ceed8a38f04e425ba8e6eb70b5227f1
Reviewed-by: Richard J. Moore <rich@kde.org>
Reviewed-by: Shane Kearns <shane.kearns@accenture.com>
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 <shane.kearns@accenture.com>