Add raw response body to Sawyer::Response

I recently had need of the raw body when working with Octokit without any of the filtering or processing from Sawyer. I ended up with a little monkey patch that essentially did what this PR does. All tests are green. 

Thanks for a great project!

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Provides access to the raw response body as `Response#body`. The addition of
the response body gives added symmetry to the reponse object since we can
get to the headers, status, env, etc.