Generate enum property if values key is passed in documentation

[Lordnibbler]

grape-entity raises an error if values: key is passed outside of the documentation hash:

expose :thing2,  values: [1, 2, 3], documentation: { type: Integer }
# => gems/grape-entity-0.5.1/lib/grape_entity/entity.rb:539:in `block in valid_options': :values is not a valid option. (ArgumentError)

However, grape-swagger-entity has business logic requiring the values: key to be passed like the above example.

This PR changes the business logic to expect the values: key inside the documentation: hash as such:

expose :thing2, documentation: { type: Integer, values: [1, 2, 3] }

[Lordnibbler]

@Bugagazavr @dmitry for review, please!

[dmitry]

I think it's ok like that. Could be merged in my opinion.

[kzaitsev]

@Lordnibbler Thanks, please rebase and add some information to CHANGELOG.md

[Lordnibbler]

@Bugagazavr @dmitry done

[kzaitsev]

Thanks

[dblock]

This should be put back :)