QtNetworkReplyEx enhancements

Some enhancements made after a quick review by Robert Knight
(robertknight@gmail.com):

* Creating fully resolved redirect url.
* Preserving QNewotkRequest Attributes from the original request
* For HEAD requests, a HEAD request is sent, for others we send a
  GET request.
* New error code UnknownRedirectError is emitted in case the
  operation (GET/HEAD etc) is not known.