core: EnumUtils - used cached map if available

Reuse the cached version of the uppercase map in EnumUtils if it already
existed.

This has two slight benefits:
1. Better memory management, by making sure we're reusing the same
   references all over.
2. It solves the normal priority FindBugs
   RV_RETURN_VALUE_OF_PUTIFABSENT_IGNORED  warning on this issue.

Change-Id: I486a09d190a2c02be666400c59795e7104f83d79
Signed-off-by: Allon Mureinik <amureini@redhat.com>