R1:60e91d393421

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…