Gzip support for active resource

http://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/2070-gzip-deflate-support-for-activeresource

At the moment ActiveResource does not support gzip/deflate. As it does
use XML as a transfer medium, this becomes highly inefficient with
large datasets.

Gzip is one of the oldest remedies for this, and all major application
servers for rails support it, but activeresource doesn't.

To enable: have ZLib installed and add this to your ActiveResource
base class:
base.headers['Accept-Encoding'] = 'deflate'