Fix crashes where transports returned errors unhandled by the message parser.

The http.Handler-based transport body reader was returning error types
not understood by the recvMsg parser. See #557 for some background and
examples.

Fix the http.Handler transport and add tests. I copied in a subset of
the http2 package's serverTest type, adapted slightly to work with
grpc. In the process of adding tests, I discovered that
ErrUnexpectedEOF was also not handled by the regular server
transport. Document the rules and fix that crash as well.

Unrelated stuff in this CL:

* make tests listen on localhost:0 instead of :0, to avoid Mac firewall
  pop-up dialogs.

* rename parser.s field to parser.r, to be more idiomatic that it's an
  io.Reader and not anything fancier. (it's not acting like type
  stream, even if that's the typical concrete type)

* move 5 byte temp buffer into parser, rather than allocating it for
  each new message. (drop in the bucket improvement in garbage; more
  to do later)

* rename http2RSTErrConvTab to http2ErrConvTab, per Qi's earlier
  CL. Also add the HTTP/1.1-required error mapping for completeness,
  not that it should ever arise with gRPC, also per Qi's earlier CL
  referenced in #557.

rpc_util_test.go

@@ -65,7 +65,7 @@ func TestSimpleParsing(t *testing.T) {
 		{append([]byte{0, 1, 0, 0, 0}, bigMsg...), nil, bigMsg, compressionNone},
 	} {
 		buf := bytes.NewReader(test.p)
-		parser := &parser{buf}
+		parser := &parser{r: buf}
 		pt, b, err := parser.recvMsg()
 		if err != test.err || !bytes.Equal(b, test.b) || pt != test.pt {
 			t.Fatalf("parser{%v}.recvMsg() = %v, %v, %v\nwant %v, %v, %v", test.p, pt, b, err, test.pt, test.b, test.err)
@@ -77,7 +77,7 @@ func TestMultipleParsing(t *testing.T) {
 	// Set a byte stream consists of 3 messages with their headers.
 	p := []byte{0, 0, 0, 0, 1, 'a', 0, 0, 0, 0, 2, 'b', 'c', 0, 0, 0, 0, 1, 'd'}
 	b := bytes.NewReader(p)
-	parser := &parser{b}
+	parser := &parser{r: b}
 
 	wantRecvs := []struct {
 		pt   payloadFormat