Tidy.
Fix leak in utiHumanReadableDescription.

It was retaining the given uti through a __bridge_retained cast,
and not releasing it.  This this by using __bridge instead.
Use __bridge instead of __bridge_retained so that we don't need
to CFRelease afterwards.  We already have a reference to the NSString,
we don't need to retain it again.
Make utiFilenameToMIME("eml") = "message/rfc822".

This is then symmetric with utiMIMEToExtension, which also
has a special case for message/rfc822.
Add UTITests.