R1:6b05d2be287e

Add a setup warning to detect "SetInputFilter DEFLATE" and other "Content-Encoding" request mangling

Summary: Ref T13507. See that task for discussion.

Test Plan: Faked different response behaviors and hit both variations of this error.

Maniphest Tasks: T13507

Differential Revision: https://secure.phabricator.com/D21116