Fix an issue with passing HTTP headers through in proxied cluster requests

Summary:
I think this fixes the Mercurial + HTTP cluster issue. PHP adds HTTP_ but we were not stripping it, so we would convert an X-Whatever-Zebra header into an Http-X-Whatever-Zebra header.

I don't think this behavior has changed? So maybe it just never worked? Git is more popular than Mercurial and SSH is easier to configure than HTTP, so it's plausible. I'll keep a careful eye on this when it deploys.

Test Plan:

• Set up local service-based Mercurial repository.
• Tried to clone, got similar error to cluster.
• Applied patch, clean clone.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D15660