This was a t.Parallel test but it was using the global DefaultTransport
via the global Get func.

Use a private Transport that won't have its CloseIdleConnections etc
methods called by other tests.

(I hit this flake myself while testing a different change.)

Change-Id: If0665e3e8580ee198f8e5f3079bfaea55f036eca
Reviewed-on: https://go-review.googlesource.com/37624
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>