io: NopCloser forward WriterTo implementations if the reader supports it
This patch also include related fixes to net/http.
io_test.go don't test reading or WritingTo of the because the logic is simple.
NopCloser didn't even had direct tests before.
Fixes #51566
Change-Id: I1943ee2c20d0fe749f4d04177342ce6eca443efe
GitHub-Last-Rev:
a6b9af4e945a6903735a74aa185e2d1c4c2e2cef
GitHub-Pull-Request: golang/go#52340
Reviewed-on: https://go-review.googlesource.com/c/go/+/400236
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>