Extended User API to expose admin and can_create_group for user creation/updating.

Also, is_admin and can_create_group are exposed in the user information.
Fixed attributes_for_keys to process properly keys with boolean values (since false.present? is false).
Added test cases to verify admin status of users created via API.
Merge pull request #5213 from bladealslayer/feature/user_api_extended

Extended User API to expose admin and can_create_group for user creation...