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.
