[TW-1604] Enumeration attributes should be more forgiving regarding capitalization

_Jens Erat on 2015-04-27T09:34:16Z says:_

With the changed priority attribute, `priority:h` cannot be entered any more (failing with "The 'priority' attribute does not allow a value of 'h'.").

I'd propose to handle enumeration attributes in a more forgiving way and accept different capitalization if the result is still unique, ie. compare the lower case values. An alternative might be some option that can be set within the UDA which makes it case-insensitive.