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.
Thinking about it again, the reason to do it this way was to handle the case where people had a clone of the mb web service but didn't have SSL - so they would want mblogin (to say localhost) without SSL, whereas a login to an official MB server should be SSL.

So this PR is probably good as it is.