Parse HTTP responses with empty "reason-phrase" properly in PhutilHTTPResponseParser

Summary: Depends on D19628. This ports the fix from that change to PhutilHTTPResponseParser, which is theoretically the future of HTTP response parsing.

Test Plan: Added unit test and made it pass; this code is currently unreachable.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19631