Add PowerEnum::Reflection::EnumerationReflection::polymorphic?() function
that just returns false for power enum reflections.

This is required (at least in Rails4) to do queries like
  where( enum: Enum[value] )
Test that both enum and enum_id finders work
Merged of https://github.com/albertosaurus/power_enum_2/pull/1

Implements ".polymorphic?" on PowerEnum::Reflection::EnumerationReflection::polymorphic?