(stable) Strip "Transfer-Encoding" headers from proxied HTTP responses

This is a likely fix for HTTP clones against proxied repositories in the
cluster, although I'm not 100% sure I'm replicating it correctly.

The issue appears to be that we're proxying all the headers, including the
"Transfer-Encoding" header, although the request will already have stripped
any encoding. This might cause us to emit a "chunked" header without a
chunked body.

Auditors: chad