Add scope to Exchange request

See https://tools.ietf.org/html/rfc6749#section-4.1.1

Author: lancerushing

oauth2.go

@@ -218,6 +218,9 @@ func (c *Config) Exchange(ctx context.Context, code string, opts ...AuthCodeOpti
 	if c.RedirectURL != "" {
 		v.Set("redirect_uri", c.RedirectURL)
 	}
+	if len(c.Scopes) > 0 {
+		v.Set("scope", strings.Join(c.Scopes, " "))
+	}
 	for _, opt := range opts {
 		opt.setValue(v)
 	}

oauth2_test.go

@@ -110,7 +110,7 @@ func TestExchangeRequest(t *testing.T) {
 		if err != nil {
 			t.Errorf("Failed reading request body: %s.", err)
 		}
-		if string(body) != "code=exchange-code&grant_type=authorization_code&redirect_uri=REDIRECT_URL" {
+		if string(body) != "code=exchange-code&grant_type=authorization_code&redirect_uri=REDIRECT_URL&scope=scope1+scope2" {
 			t.Errorf("Unexpected exchange payload; got %q", body)
 		}
 		w.Header().Set("Content-Type", "application/x-www-form-urlencoded")
@@ -154,7 +154,7 @@ func TestExchangeRequest_CustomParam(t *testing.T) {
 		if err != nil {
 			t.Errorf("Failed reading request body: %s.", err)
 		}
-		if string(body) != "code=exchange-code&foo=bar&grant_type=authorization_code&redirect_uri=REDIRECT_URL" {
+		if string(body) != "code=exchange-code&foo=bar&grant_type=authorization_code&redirect_uri=REDIRECT_URL&scope=scope1+scope2" {
 			t.Errorf("Unexpected exchange payload, %v is found.", string(body))
 		}
 		w.Header().Set("Content-Type", "application/x-www-form-urlencoded")
@@ -200,7 +200,7 @@ func TestExchangeRequest_JSONResponse(t *testing.T) {
 		if err != nil {
 			t.Errorf("Failed reading request body: %s.", err)
 		}
-		if string(body) != "code=exchange-code&grant_type=authorization_code&redirect_uri=REDIRECT_URL" {
+		if string(body) != "code=exchange-code&grant_type=authorization_code&redirect_uri=REDIRECT_URL&scope=scope1+scope2" {
 			t.Errorf("Unexpected exchange payload, %v is found.", string(body))
 		}
 		w.Header().Set("Content-Type", "application/json")