Write CONNECT response in a single call to Conn.Write() (#117)
This introduces a few changes from the previous version:
1. If we build an http.Response and call its Write() function, it will
write the response line-by-line. This seems to be the cause of some
flakiness with clients that start tunnel mode after the status line,
rather than after the response header, as per RFC9110 9.3.6.
2. We only support HTTP 1.0 and 1.1. We no longer just blindly copy over
the HTTP version from the client request.
3. The status code is still 200 but the status message has changed from
"OK" to "Connection Established", in line with what other proxies are
doing (I tested Squid, Martian and Zscaler). Clients ignore this
anyway, going with the status code, so this shouldn't matter.
4. We no longer send a 'Connection: close' header with the response.
This is redundant in a CONNECT request, and follows what other
proxies (Squid, Martian and Zscaler) do.
