Disable automatic decoding of "Content-Encoding" responses during "Accept-Encoding" setup test

Summary:
Ref T13507. Now that we handle processing of "Content-Encoding: gzip" headers by default, this setup check can get a decompressed body back. Since it specifically wants a raw body back, disable this behavior.

Also, "@" a couple things which can get in the way if they fail now that error handling is more aggressive about throwing on warnings.

Test Plan: Ran setup check after other changes in T13507, got clean result.

Maniphest Tasks: T13507

Differential Revision: https://secure.phabricator.com/D21122