Fix 1373, Improve EDDL() Exception message in the [Flags] case
- https://aspnetwebstack.codeplex.com/workitem/1373
- "Error message is not clear when enum that has flags attribute calls EnumDropDownListFor html helper"
- Add new message for case where ModelType has a [Flags] attribute
- Provide EnumHelper.HasFlags(Type) internal method to make determination easy for EDDL()
- Leads to a duplicate call to Nullable.GetUnderlyingType(Type) but only in the failure case
Merge branch 'fix1373'