(stable) Don't combine automatic output compression with "Content-Length"

Summary:
Fixes T12013. Send either "Content-Length" or enable output compression, but not both.

Prefer compression for static resources (CSS, JS, etc).

Test Plan: Ran curl -v ..., no longer saw responses with both compression and Content-Length.

Reviewers: chad, avivey

Reviewed By: avivey

Subscribers: avivey

Maniphest Tasks: T12013

Differential Revision: https://secure.phabricator.com/D17045