net/http: fix a long test after CL 185117

The net/url error in question now quotes the URL, so update the expected
output string.

While at it, also update a comment in httputil, though that doesn't
affect any test.

Fixes #33910.

Change-Id: I0981f528b24337c2952ef60c0db3b7ff72d72110
Reviewed-on: https://go-review.googlesource.com/c/go/+/192078


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>