net/http/httputil: ReverseProxy FlushInterval no longer flushes headers in go1.12 [1.12 backport]

[gopherbot]

@bradfitz requested issue #31125 to be considered for backport to the next 1.12 minor release.

@gopherbot, please backport to Go 1.12.

[gopherbot]

Change https://golang.org/cl/170137 mentions this issue: [release-branch.go1.12] net/http/httputil: make ReverseProxy flush headers on FlushInterval

[gopherbot]

Closed by merging ec06e9b to release-branch.go1.12.