HTTP POST body support, chunked encoding, suppression of miscellaneous headers, disable gzip, ...

[mzpqnxow]

Would be nice to have HTTP POST body support (chunked or standard/identity format) as well as a few more simple flag-based tunables for the HTTP module:

• Flag to disable gzip encoding advertisement (DisableCompression)
• Flag to exclude Accept header
• Flag to explicitly set the Content-Type directly

I've been using the referenced PR for a little while, mainly for the POST body bit. It works, but I'm not a great golang dev so it would be great if someone could review this. I don't think it's horrible, but there's probably room for improvement. I also did not exhaustively test it with unusual or invalid combinations of flags, or invalid values