Add option to skip check for chunking plus content-length

[jdforsythe]

There are a bunch of examples of servers (incorrectly) sending both Transfer-Encoding: chunked and Content-Length: headers (e.g. request/request#2091).

This parser checks for the existence of both headers and gives the HPE_UNEXPECTED_CONTENT_LENGTH error if they both exist.

There should be an option to skip this check if desired