The mod_h2 workaround CL (git rev 28d1bd4f,
https://golang.org/cl/25362) introduced a regression where the
Transport could write two DATA frames, both with END_STREAM, if the
Request.Body returned (non-0, io.EOF). strings.Reader, bytes.Reader
are the most common Reader types used with HTTP requests and they only
return (0, io.EOF) so we got generally lucky and things seemed to
work, but other Readers do return (non-0, io.EOF), like the HTTP
transport/server Readers. This is why we broke the HTTP proxy code,
when proxying to HTTP/2.

Updates golang/go#16788 (fixes after it's bundled into std)

Change-Id: I42684017039eacfc27ee53e9c11431f713fdaca4
Reviewed-on: https://go-review.googlesource.com/27406
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Chris Broadfoot <cbro@golang.org>