Support HTTP basic auth in HTTPFuture

Summary: Ref T4590. We already sort of support this for Conduit, push the support a little bit deeper.

Test Plan: Added basic auth across my entire nginx. Requests with no auth or with the wrong auth failed; requests with the right auth worked.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4590

Differential Revision: https://secure.phabricator.com/D8621