Allow using annotate with multiple enumerated_attributes

Fixes https://github.com/jeffp/enumerated_attribute/issues/56

This is difficult to provide a RSpec test for (I tried), but the fix does work and permits running annotate against models that contain multiple enumerated_attributes.

This fix was inspired by https://github.com/jeffp/meta_programming/blob/master/lib/meta_programming/object.rb#L42