Fixes golang/oauth2#276

Change-Id: I3f2bed7201f2e6bb58913f3fae2e64287b9a66d5
Reviewed-on: https://go-review.googlesource.com/c/161959


Reviewed-by: Ross Light <[email protected]>
Reviewed-by: Jean de Klerk <[email protected]>
Reviewed-by: Chris Broadfoot <[email protected]>
Run-TryBot: Brad Fitzpatrick <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>