oauth2: fix warning from go vet
oauth2: fix warning from go vet

go vet reports the following warning:
oauth2.go:298: golang.org/x/oauth2/internal.ErrorTransport \
composite literal uses unkeyed fields

Change-Id: I576234fcaedb745702fdf8c3431e55128788079d
Reviewed-on: https://go-review.googlesource.com/24991
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/oauth2