[release-branch.go1.12] net/http: prevent Transport from spamming stderr on server 408 reply

HTTP 408 responses now exist and are seen in the wild (e.g. from
Google's GFE), so make Go's HTTP client not spam about them when seen.
They're normal (now).

Fixes #32367
Updates #32310

Change-Id: I558eb4654960c74cf20db1902ccaae13d03310f6
Reviewed-on: https://go-review.googlesource.com/c/go/+/179457


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
(cherry picked from commit ba66d89d)
Reviewed-on: https://go-review.googlesource.com/c/go/+/181239


Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>