Clarify resetting of exxon body reader.

- Use a more intention-revealing name.
- Add an explanatory comment since it's such
  an odd case.
- Remove `if request_params[:response_block]`
  conditional. We always want the `NonStreamingResponseBodyReader`
  in this case. If there's no response block, the
  response body reader will already be set to this, but
  re-assigning it causes no harm, so there's no
  need for the conditional at all.