always return Content-Length: 0 if r is not set

Main PR: https://github.com/mitmproxy/netlib/pull/26

Always add a Content-Length header unless the <code>raw</code> flag is set. This is necessary for responses to be conformant to the [specs](http://tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-16#section-3.3):
<blockquote>6.  Otherwise, this is a response message without a declared message-body length, so the message-body length is determined by the number of octets received prior to the server closing the connection.
</blockquote>
pathod usually keeps the connection alive, so the requests would hang otherwise. After all, the Content-Length header can still be omitted by passing the r flag.