net/http: ServeContent()/ServeFile() doesn't return expected response when WriteTimeout happens [1.15 backport]

[gopherbot]

@odeke-em requested issue #43822 to be considered for backport to the next 1.15 minor release.

As per the comment by Ian in #43822 (comment), this issue is a worthy candidate. @gopherbot please backport this issue.

[toothrot]

Approved. This is a serious issue with no workaround.

[gopherbot]

Change https://golang.org/cl/296530 mentions this issue: [release-branch.go1.15] internal/poll: netpollcheckerr before sendfile

[gopherbot]

Closed by merging 30357d6 to release-branch.go1.15.