Update FilterPaneTagLib.groovy

Fixed problem that caused IllegalArgumentException while enum value was not set. The exception was caught silently so it was not obvious that it was thrown. I saw it while debugging.

Also fixed problem that valueMessagePrefix was not used for enum values unless values were explicitly specified. Removing the optionValue from ctrlAttrs made the trick.
Merge pull request #38 from stenix71/patch-7

Update FilterPaneTagLib.groovy