net/http/httputil: don't insert default User-Agent header in proxied requests
When ReverseProxy forwards a request with no User-Agent header, leave
the header in the forwarded request blank rather than inserting the
default Go HTTP clent User-Agent.
We already did this for NewSingleHostReverseProxy; generalize it to
every ReverseProxy.
Change-Id: Id81a230cb8d384acdfae190b78a4265d80720388
Reviewed-on: https://go-review.googlesource.com/c/go/+/407375 Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Michael Knyszek <mknyszek@google.com>