Tweak url formatting

See http://forums.musicbrainz.org/viewtopic.php?id=5394 for original issue (with discogs) and proposed fix.

This tweak goes slightly beyond this, only providing a specific port if it is not 80 or 443.

Thanks to DrunkenDwarf for his proposed fix which inspired this PR.
To answer my own question after a look into the code: It is a workaround / special case for the MB host, since the port is set to 80 by default in the settings. Another approach would be to set the port to 443 by default.

But in the context of this PR this is fine. Nice additional improvement is that now one can actually use HTTPs for non-MB API calls as long as the HTTPs standard port is being used.