Skin: Make normalizeKey() accept non-canonical case variants of keys
For example, passing 'MonoBook' and 'monobook' will now both yield
'monobook'; previously, the former yielded the default skin ('vector').
This magically fixes casing issues in the &useskin= URL parameter, as
well as in configuration variables like $wgDefaultSkin.
(This is per agreement on wikitech-l discussion a few weeks ago,
"What should be the recommended / supported way to do skins?").
Change-Id: I0df77b6d42726605b56d52b34e204113e8089e80