First stab at characterising http.client.HTTPConnnection.send().

https://github.com/moreati/bpo-23740

This uses a webserver that returns request details, in the body of the response. Raw (TCP level) content is included. It shares a similar purpose to HTTP TRACE command. In principal the bytes that HTTPConection.send() writes will match to the bytes returned (after they're decapsulated from the JSON). I've not tested that aspect yet.

TODO
- further testing (verify round trip, bytes in = bytes out) 
- cover multiple Python versions
- cover cases such client manually setting Content-Length