Introduced by golang.org/cl/31447.

grpc-go does concurrent Read/Close calls to the Request.Body. Since
that was supported previously, continue to support it, and add a test.

Updates grpc/grpc-go#938

Change-Id: I8a5cf6b28c5eca346ea46c4129021172e489f71d
Reviewed-on: https://go-review.googlesource.com/31636
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>