KIJIHIVE-53. Fix handling of generic enums.

GenericEnumSymbols should be handled the same way as specific enums. Rather than
type casting to a String which could cause problems in weird corner cases,
simply use the .toString() method of the underlying object which should insulate
us from any ClassCastExceptions.

Review: http://review.kiji.org/r/2066/
Reviewer: lsheng