Make the CONNECT Host header the same as the Request-URI.

It's possible to construct a request where the Host header differs from
the authority in the URL, for example in an extension with
nsIHttpChannel and setRequestHeader. MakeConnectString generates a
host:port string for the CONNECT Request-Line, but peeks into the
tunneled request in order to copy the Host header to the proxy request.

Instead, use the same host:port string for Host as is used in the
Request-URI, to avoid revealing the plaintext of the Host header outside
of the tunnel.

Backport of https://hg.mozilla.org/mozilla-central/rev/a1f6458800d4.