Support for rfc 7523 in refresh token

Author: sks

Diff for: oauth2.go

@@ -246,10 +246,11 @@ func (c *Config) Client(ctx context.Context, t *Token) *http.Client {
 // automatically refreshing it as necessary using the provided context.
 //
 // Most users will use Config.Client instead.
-func (c *Config) TokenSource(ctx context.Context, t *Token) TokenSource {
+func (c *Config) TokenSource(ctx context.Context, t *Token, opts ...AuthCodeOption) TokenSource {
 	tkr := &tokenRefresher{
 		ctx:  ctx,
 		conf: c,
+		opts: opts,
 	}
 	if t != nil {
 		tkr.refreshToken = t.RefreshToken
@@ -266,6 +267,7 @@ type tokenRefresher struct {
 	ctx          context.Context // used to get HTTP requests
 	conf         *Config
 	refreshToken string
+	opts         []AuthCodeOption
 }
 
 // WARNING: Token is not safe for concurrent access, as it
@@ -277,10 +279,15 @@ func (tf *tokenRefresher) Token() (*Token, error) {
 		return nil, errors.New("oauth2: token expired and refresh token is not set")
 	}
 
-	tk, err := retrieveToken(tf.ctx, tf.conf, url.Values{
+	v := url.Values{
 		"grant_type":    {"refresh_token"},
 		"refresh_token": {tf.refreshToken},
-	})
+	}
+	for _, opt := range tf.opts {
+		opt.setValue(v)
+	}
+
+	tk, err := retrieveToken(tf.ctx, tf.conf, v)
 
 	if err != nil {
 		return nil, err