Support "Range: bytes=123-" requests

Summary:
Ref T12219. We currently only support Range requests like "bytes=123-456", but "bytes=123-", meaning "until end of file", is valid, and Chrome can send these requests.

I suspect this is the issue with T12219.

Test Plan: Used nc local.phacility.com 80 to pipe raw requests, saw both "bytes=123-456" and "bytes=123-" requests satisfied correctly.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12219

Differential Revision: https://secure.phabricator.com/D17626