net/http/httputil: fix goroutine leak for DumpRequestOut

When an invalid URL was passed to DumpRequestOut, it would directly return
without gracefully shutting down the reader goroutine.

So we create a channel and signal the reader goroutine to exit
if an error occurs during roundtrip.

Fixes #32571

Change-Id: I8c2970f1601e599f3d1ebfed298faf5f5716fc2c
Reviewed-on: https://go-review.googlesource.com/c/go/+/182037


Run-TryBot: Agniva De Sarker <agniva.quicksilver@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>