1. use CONNECT instead of GET.
CONNECT has exactly the meaning
we want; most of the time people
connect to ip:port; we're connecting
to /_goRPC_
2. wait for a successful HTTP response
before assuming we are connected to
the RPC protocol. this allows better
error reporting and also keeps the
protocol alternating between speakers,
so that the buffering in the HTTP request
reader cannot accidentally eat some
RPC bytes too.
gotest sometimes hangs, but not in HTTP.
gotest -match=Unknown hangs every few runs
even in a clean client.