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.

R=r
DELTA=117  (57 added, 44 deleted, 16 changed)
OCL=31656
CL=31685