oauth: delete session key instead of set to empty

Adds the Delete function to the Session wrapper so that the Macaron
function for deleting keys from a Session can be used.

https://go-macaron.com/docs/middlewares/session#implement-provider-interface

pkg/api/login.go

@@ -36,7 +36,7 @@ func LoginView(c *middleware.Context) {
 	viewData.Settings["disableLoginForm"] = setting.DisableLoginForm
 
 	if loginError, ok := c.Session.Get("loginError").(string); ok {
-		c.Session.Set("loginError", "") // TODO: is there a proper way to delete a session var?
+		c.Session.Delete("loginError")
 		viewData.Settings["loginError"] = loginError
 	}
 

pkg/middleware/session.go

@@ -93,6 +93,8 @@ type SessionStore interface {
 	Set(interface{}, interface{}) error
 	// Get gets value by given key in session.
 	Get(interface{}) interface{}
+	// Delete deletes a key from session.
+	Delete(interface{}) interface{}
 	// ID returns current session ID.
 	ID() string
 	// Release releases session resource and save data to provider.
@@ -128,6 +130,13 @@ func (s *SessionWrapper) Get(k interface{}) interface{} {
 	return nil
 }
 
+func (s *SessionWrapper) Delete(k interface{}) interface{} {
+	if s.session != nil {
+		return s.session.Delete(k)
+	}
+	return nil
+}
+
 func (s *SessionWrapper) ID() string {
 	if s.session != nil {
 		return s.session.ID()