Use `defined_enums` to fetch collection from enum

By using `defined_enums` instead of `.send(pluralized_method)`, only one
method is needed to implement enum support with form objects.
Merge pull request #1113 from gdott9/defined_enums

Add support for Reform gem
We've done this by relying only on `defined_enums` Hash for enum collections, instead of dynamically defined class methods like Post.statuses.