Support writing a raw HTTP body in HTTPSFuture

Summary: Ref T7019. We need this to proxy the underlying Git/Mercurial/etc requests.

Test Plan: Proxied requests; see next revision.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7019

Differential Revision: https://secure.phabricator.com/D11493