MINOR: http/hardening: implement hardening "reject-content-length-with-te"

MINOR: http/hardening: implement hardening "reject-content-length-with-te"

Rejects requests and responses having both a Content-Length header
field and a Transfer-Encoding header field. While these are permitted
by HTTP/1.1 (Transfer-Encoding wins), it can cause problems when the
message first passed through an HTTP/1.0 gateway that does not
recognize Transfer-Encoding, and can desynchronize this gateway from
the rest of the communication. These are still seldom seen in legacy
client or server applications, sometimes in relation with compression
and/or caching. While HAProxy properly normalizes them, it is generally
safe to reject them on public facing listeners.