Puller was getting confused receiving JSON-only docs (no attachments) with gzip response encoding

It was looking at the “Content-Encoding: gzip” header and thinking that this was a JSON _part_ that
was encoded, so it tried to decode it itself. Whereas the top-level gzip decoding is done already by
NSURLConnection. So it was trying to gzip-decode plaintext, oops.
Fixes #281