Handle the state parameter in callback URL.

This fixes login with certain providers, e.g. Authentik,
that trigger the message
"OAuth: State not found in session, are you sure you stored it?"

This closes #6.

Generic.php

@@ -10,6 +10,10 @@ use OAuth\Common\Http\Uri\Uri;
  */
 class Generic extends AbstractOAuth2Base
 {
+    /** @inheritdoc */
+    public function needsStateParameterInAuthUrl() {
+        return true;
+    }
 
     /** @inheritdoc */
     public function getAuthorizationEndpoint()