Make Web API response headers accessible

The response headers in Slack's Web API contain information that is sometimes useful. For example x-oauth-scopes is a list of scopes that the token has. This is the only way (that I know of) to check the scopes on an existing token through the API. My use case is I'd like to be able to use auth.test to check the scopes on a token via x-oauth-scopes.

The current implementation doesn't give a way to access the response headers on a successful request, since it returns only the body:
https://github.com/slack-ruby/slack-ruby-client/blob/ebf98319cf9d89ad4e75dbca0ae8ecf94a855aa3/lib/slack/web/faraday/request.rb#L36

The headers are accessible on the error object raised on failed requests since the entire response object is on the error:

slack_error.response.headers

I'm trying to think how we could make the headers accessible on successful responses. Some bad ideas:

1. Shove them into the body object under the key response_headers. This object is then wrapped in Slack::Messages::Message and returned. I don't like this because it pollutes the body.
2. An option that can be passed to any request (possibly also set on a client) that causes it to return something different:
   1. The entire raw response object, giving access to the body as well as the headers (and much else).
   2. [body, response_headers]
   3. etc.

Start with lib/slack/web/faraday/request.rb at the linked return statement, then read the issue discussion about exposing successful-response headers. A complete change would make headers accessible without polluting the response body, but the issue does not settle the API shape or name tests to run.