handle proxy requests better

[bagder]

GET http://example.com/this HTTP/1.1
Host: example.com
User-Agent: moo/2000

We don't know where the proxy is so we can't generate a perfect command line, but we can do better than now...

$ ./h2c < test-proxy
curl --http1.1 --header Accept: --user-agent "moo/2000" https://example.comhttp://example.com/this