auth_headers are not merged into response.headers if is_batch_request

if is_batch_request, auth_headers are generated but they are not merged into response headers.

https://github.com/lynndylanhurley/devise_token_auth/blob/master/app/controllers/devise_token_auth/concerns/set_user_by_token.rb#L104

merging auth_headers should be in any cases.

Start in app/controllers/devise_token_auth/concerns/set_user_by_token.rb around line 104, where auth_headers are generated for batch requests. Trace how response.headers is handled in both batch and non-batch cases, and verify that auth_headers are merged in every case.

Written by the indexing model from the issue text.