Update QTBUG-17223 for Qt 4.8

Update QTBUG-17223 for Qt 4.8

In Qt 4.7, http network requests are assigned to http connection
channels before connecting the channel.
In Qt 4.8, channels are connected "blind" as this gives a performance
improvement in certain circumstances.

On the assumption that User-Agent should be the same for all the
requests being sent to the server in a given burst, we use the first
queued request to set the user agent for a http proxy.

Task-number: QTBUG-17223
Reviewed-by: Markus Goetz
Reviewed-by: Martin Petersson