When FlushInterval is specified on ReverseProxy, the ResponseWriter is
wrapped with a maxLatencyWriter that periodically flushes in a
goroutine. That goroutine was not being cleaned up at the end of the
request. This resulted in a panic when Flush() was being called on a
ResponseWriter that was closed.

The code was updated to always send the done message to the flushLoop()
goroutine after copying the body. Futhermore, the code was refactored to
allow the test to verify the maxLatencyWriter behavior.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/6033043