Add unit test to ensure %3A in a path isn't decoded to ':'

QUrl("http%3A%2F%2Fexample.com") has only a path of
"http%3A%2F%2Fexample.com". In Qt 5.0 and 5.1, the %3A would get decoded
to ':', which in turn makes the URL invalid (colon before first slash).

Found via discussion on the interest mailing list.

Change-Id: I7f4f242b330df280e635eb97cce123e742aa1b10
Reviewed-by: David Faure <david.faure@kdab.com>