net/http/httputil: flush ReverseProxy immediately if Content-Length is -1

Finish up a prior TODO by making ReverseProxy flush immediately
if Content-Length is -1, which is a case that can occur if for
example we have a streamed response, or chunked encoding, or when
the body's length wasn't known.

Fixes #41642

Change-Id: I30babaaf3e14837b99e3ecdc562a0a0e50c579bf
GitHub-Last-Rev: efc019a9fe361082d40bee77317018c3b80451a3
GitHub-Pull-Request: golang/go#41858
Reviewed-on: https://go-review.googlesource.com/c/go/+/260637


Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Ian Lance Taylor <iant@golang.org>
Trust: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>