Previously when RoundTrip returned a non-nil error, the proxy returned a
StatusBadGateway error, instead of first calling ModifyResponse. This
commit first calls ModifyResponse, whether or not the error returned
from RoundTrip is nil.

Also closes response body when ModifyResponse returns an error. See #22658.

Fixes #21255

Change-Id: I5b5bf23a69ae5608f87d4ece756a1b4985ccaa9c
Reviewed-on: https://go-review.googlesource.com/54030Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>