[release-branch.go1.12] net/http/httputil: make ReverseProxy flush headers on FlushInterval

A regression was introduced in CL 137335 (5440bfc2) that caused FlushInterval
to not be honored until the first Write() call was encountered. This change
starts the flush timer as part of setting up the maxLatencyWriter.

Fixes #31144

Change-Id: I75325bd926652922219bd1457b2b00ac6d0d41b0
Reviewed-on: https://go-review.googlesource.com/c/go/+/170066


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
(cherry picked from commit 2cc34738)
Reviewed-on: https://go-review.googlesource.com/c/go/+/170137