oauth2: avoid exporting Exchange, use NewTransportWithCode.

oauth2.go

Index: oauth2.go 
diff --git a/oauth2.go b/oauth2.go 
index b87de7d66233ee258b7d656b92aa899aa6b4127c..edfd75de35c095098ca376c991cb0cfcafb18ce5 100644 
--- a/oauth2.go 
+++ b/oauth2.go 
@@ -172,7 +172,7 @@ func (c *Config) NewTransport() Transport { 
 // it succesffully retrieves a new token, creates a new transport 
 // authorized with it. 
 func (c *Config) NewTransportWithCode(exchangeCode string) (Transport, error) { 
-	token, err := c.Exchange(exchangeCode) 
+	token, err := c.exchange(exchangeCode) 
	if err != nil { 
	return nil, err 
	} 
@@ -186,22 +186,6 @@ func (c *Config) NewTransportWithCache(cache Cache) (Transport, error) { 
	return NewAuthorizedTransportWithCache(c, cache) 
 } 
 
-// Exchange exchanges the exchange code with the OAuth 2.0 provider 
-// to retrieve a new access token. 
-func (c *Config) Exchange(exchangeCode string) (*Token, error) { 
-	token := &Token{} 
-	err := c.updateToken(token, url.Values{ 
-	"grant_type": {"authorization_code"}, 
-	"redirect_uri": {c.opts.RedirectURL}, 
-	"scope": {strings.Join(c.opts.Scopes, " ")}, 
-	"code": {exchangeCode}, 
-	}) 
-	if err != nil { 
-	return nil, err 
-	} 
-	return token, nil 
-} 
- 
 // FetchToken retrieves a new access token and updates the existing token 
 // with the newly fetched credentials. If existing token doesn't 
 // contain a refresh token, it returns an error. 
@@ -240,6 +224,22 @@ func (c *Config) validate() error { 
	return nil 
 } 
 
+// Exchange exchanges the exchange code with the OAuth 2.0 provider 
+// to retrieve a new access token. 
+func (c *Config) exchange(exchangeCode string) (*Token, error) { 
+	token := &Token{} 
+	err := c.updateToken(token, url.Values{ 
+	"grant_type": {"authorization_code"}, 
+	"redirect_uri": {c.opts.RedirectURL}, 
+	"scope": {strings.Join(c.opts.Scopes, " ")}, 
+	"code": {exchangeCode}, 
+	}) 
+	if err != nil { 
+	return nil, err 
+	} 
+	return token, nil 
+} 
+ 
 func (c *Config) updateToken(tok *Token, v url.Values) error { 
	v.Set("client_id", c.opts.ClientID) 
	v.Set("client_secret", c.opts.ClientSecret)