net/http: make Transport ignore 408 timeout messages from server [1.11 backport]

[gopherbot]

@bradfitz requested issue #32310 to be considered for backport to the next 1.11 minor release.

@gopherbot, please backport.

[dmitshur]

Per backport policy, I'm not going to accept this cherry-pick candidate for 1.11 because it's not a serious issue, and there exist two workarounds: update to Go 1.12.6 (which will have this fix), or use log.SetOutput(filteringWriter{}) with a custom filteringWriter implementation as @bradfitz suggested in #32367 (comment).