ZF-1819: Support gzipped responses

Description

We should optionally allow the client to request a gzipped response from the GD client library. Architecture of this has to be investigated (service object, http client, etc).

In order to support getting a gzipped response, 'gzip' needs to be in the user-agent, but an accept-encoding header is also required to enable gzip.

Comments

This was resolved in svn rev 9846, 9849 along with the secure authsub support.

If the zlib module is installed (gzinflate function exists), and setGzipEnabled(true) is called, then an Accept-encoding: gzip header is added and the user-agent is modified to include the word 'gzip' (as required by Google's servers).

By default, gzipped responses are off.

Marking as fixed for the 1.6.0 release.