Updates http2 to x/net git rev 0e6d34ef942 for https://golang.org/cl/18472
which means we'll get to delete a ton of grpc-go code and just use the
standard library's HTTP client instead.

Also, the comments in this CL aren't entirely accurate it turns out.
RFC 2616 says:

"The Trailer header field can be used to indicate which header fields
are included in a trailer (see section 14.40)."

And 14.40:

"  An HTTP/1.1 message SHOULD include a Trailer header field in a
   message using chunked transfer-coding with a non-empty trailer. Doing
   so allows the recipient to know which header fields to expect in the
   trailer.

   If no Trailer header field is present, the trailer SHOULD NOT include
   any header fields. See section 3.6.1 for restrictions on the use of
   trailer fields in a "chunked" transfer-coding."

So it's really a SHOULD more than a MUST.

And gRPC (at least Google's server) doesn't predeclare "grpc-status"
ahead of time in a Trailer Header, so we'll be lenient. We were too
strict anyway. It's also not a concern for the Go client we have a
different place to populate the Trailers, and it won't confuse clients
which aren't looking for them. The ResponseWriter server side is more
complicated (and strict), though, since we don't want to widen the
ResponseWriter interface. So the Go server still requires that you
predeclare Trailers.

Change-Id: Ia2defc11a2469fb8570ecfabb8453537121084eb
Reviewed-on: https://go-review.googlesource.com/18473
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>