Maps enums to database values before storing in object_changes

[cbarton]

Keep consistency between versions with regard to changes and
object_changes and how enum columns store their values.

Before, changes would map the changed attributes enum columns to the database
values (integer values). This allows reifying that version to maintain the
integrity of the enum. It did not do so for object_changes and thus, 0
for non-json columns, and the enum value for json columns would be stored instead.
For the non-json columns, it mapped any non-integer enum value to 0 because
during serialization that column is an integer. Now this is fixed,
so that object_changes stores the enum mapped value.

Here is an example:

class PostWithStatus < ActiveRecord::Base
  has_paper_trail
  enum status: { draft: 0, published: 1, archived: 2 }
end

post = PostWithStatus.new(status: :draft)
post.published!
version = post.versions.last

# Before
version.changeset #> { 'status' => ['draft', 'draft'] } (stored as [0, 0])

# After
version.changeset #> { 'status' => ['draft', 'published'] } (stored as [0, 1])

[jaredbeck]

Looks good, Chris, thanks!