Prohibited and unnecessary Connection header

[mr-bronson]

In HTTP/2, the Connection header is prohibited, but it still shows up when you look at response.request.headers for requests where response.http_version == "HTTP/2".
The Connection header is added by _client.py along with three others (Accept, Accept-Encoding, User-Agent) without asking for these headers explicitly.
Even in HTTP/1.1, the value which _client.py sets ("keep-alive") is totally useless because that's the default anyway, if the header is omitted.
Thankfully I found I could omit all these headers with client.headers.clear().