Investigate restoring streaming request body through to upstream

[mwhitworth]

This functionality was effectively removed to fix #175 - the new version of this functionality should fulfil two requirements:

• it should be tested to work with each adapter - {:stream, _} as a input value for the upstream body was supported by HTTPoison, but it was unclear (and untested) whether this was supported by the Tesla adapter at all
• as #175, we need to forward conn whenever we use Plug.Conn.read_body - but that seems to be at odds with passing in a stream to the library. Perhaps Stream.resource, and sending the latest conn to ourselves?