http: improve RFC 7230 conformance
We need to favor "Transfer-Encoding: chunked" over
"Content-Length" in the request header if they both exist.
Furthermore, we now reject redundant chunking and cases where
"chunked" is not the final encoding.
We currently do not and have no plans to decode "gzip",
"deflate", or "compress" encoding as described by RFC 7230.
That's a job more appropriate for middleware, anyways.
cf. https://tools.ietf.org/html/rfc7230
https://www.rfc-editor.org/errata_search.php?rfc=7230