Fix crash on closed connection while recv in body

Hackney may discover the connection has been closed while being in the
response body, after which an `{error, {closed, Acc}}` error is
returned:
https://github.com/benoitc/hackney/blob/master/src/hackney_client/hackney_response.erl#L301-L311

This commit makes esputnik expect that possibility and avoid crashing in
a tight loop.
Merge pull request #7 from heroku/fix-closed-conns-on-body-recv

Fix crash on closed connection while recv in body