google: make JWTConfigFromJSON set TokenURL from the JSON's token_uri

Fixes golang/oauth2#199.

Change-Id: I534def935c7143e4276b5d880127b0af35409f9a
Reviewed-on: https://go-review.googlesource.com/28411
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

google/google.go

@@ -89,6 +89,7 @@ func JWTConfigFromJSON(jsonKey []byte, scope ...string) (*jwt.Config, error) {
 		Email        string `json:"client_email"`
 		PrivateKey   string `json:"private_key"`
 		PrivateKeyID string `json:"private_key_id"`
+		TokenURL     string `json:"token_uri"`
 	}
 	if err := json.Unmarshal(jsonKey, &key); err != nil {
 		return nil, err
@@ -98,7 +99,10 @@ func JWTConfigFromJSON(jsonKey []byte, scope ...string) (*jwt.Config, error) {
 		PrivateKey:   []byte(key.PrivateKey),
 		PrivateKeyID: key.PrivateKeyID,
 		Scopes:       scope,
-		TokenURL:     JWTTokenURL,
+		TokenURL:     key.TokenURL,
+	}
+	if config.TokenURL == "" {
+		config.TokenURL = JWTTokenURL
 	}
 	return config, nil
 }