google.NewCloudSDK fails to decode credentials file if it contains
service account.
NOTE: the private key in testdata is generated only for this test.
Fixes#86 (https://github.com/golang/oauth2/issues/86).
Change-Id: I9e9e8e10763723d1bf1f953b491aa6e6f3ee760c
Reviewed-on: https://go-review.googlesource.com/4220
Reviewed-by: Andrew Gerrand <adg@golang.org>