inflate-upstream: support gzipped/deflated upstreams

Adds suport for silently inflating compressed upstreams. This is
good for when the link to the upstream is low bandwidth

This should fix https://github.com/pagespeed/ngx_pagespeed/issues/140
Merge pull request #190 from pagespeed/oschaaf-inflate-upstream

Adds suport for silently inflating compressed upstreams. This is
good for when the link to the upstream is low bandwidth

This should fix https://github.com/pagespeed/ngx_pagespeed/issues/140