Reject requests that contain both Content-Length and Transfer-Encoding

[clue]

A request message that contains both a Content-Length and Transfer-Encoding header should not happen in practice. As per https://tools.ietf.org/html/rfc7230#section-3.3.3 this ought to be handled as an error.

If a message is received with both a Transfer-Encoding and a
Content-Length header field, the Transfer-Encoding overrides the
Content-Length. Such a message might indicate an attempt to
perform request smuggling (Section 9.5) or response splitting
(Section 9.4) and ought to be handled as an error."

Refs #137
Build on top of #316