google: set expiry on JWTAccessTokenSource

Change-Id: Ib1ff6511ce38e17711486743601241285a34f164
Reviewed-on: https://go-review.googlesource.com/14934
Reviewed-by: Julien Boeuf <jboeuf@google.com>
Reviewed-by: Andrew Gerrand <adg@golang.org>
diff --git a/google/jwt.go b/google/jwt.go
index 804d9d0..6f7ec3a 100644
--- a/google/jwt.go
+++ b/google/jwt.go
@@ -67,5 +67,5 @@
 	if err != nil {
 		return nil, fmt.Errorf("google: could not encode JWT: %v", err)
 	}
-	return &oauth2.Token{AccessToken: msg, TokenType: "Bearer"}, nil
+	return &oauth2.Token{AccessToken: msg, TokenType: "Bearer", Expiry: exp}, nil
 }