gzip additional types

[jhorman]

By default only html will be gzipped. I imagine lots of heroku customers are using it to deploy api's. It would be great to have json, xml, others, enabled.

gzip_types text/plain text/css application/json application/javascript application/x-javascript text/javascript text/xml application/xml application/rss+xml application/atom+xml application/rdf+xml image/svg+xml;