[patch] HTTP request to a host behind a proxy failed due to host name lookup failure

I want to make HTTP request to a server which is behind a proxy, and request is always failed with QNetworkReply::HostNotFoundError error.
This happens because the host name can't be resolved in QHttpNetworkConnectionPrivate::startHostInfoLookup() when network layer preference (IPv4 or IPv6) is checked.
Note, in my case host name can't be resolved via DNS, and should be resolved by proxy.

This works fine on QT 4.7.x, so it's a regression in QT 5.0.0

The solution is to check layer preference based on proxy host address (if proxy is set for that http connection), and not based on destination host address.

Proposed patch is attached.