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.
This looks wrong to me, why is (and was) here a check for `port == 80` if it is HTTPs?