Fix duplicate font-weight property

Follows-up faa2bbc0d0.
Merge pull request #38 from Krinkle/patch-1

Fix duplicate font-weight property
Consider language tags as case insensitive

See https://bugzilla.wikimedia.org/show_bug.cgi?id=55666
Merge branch 'master' of github.com:wikimedia/jquery.webfonts
Handler the language being null case while converting case