If client does not provided User-Agent header, do not set default one
used by net/http package when doing request to backend.

Fixes #15524

Change-Id: I9a46bb3b7ec106bc7c3071e235b872d279994d67
Reviewed-on: https://go-review.googlesource.com/23089
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>