allow translateOrDefault with app locale

src/Translatable/Translatable.php

@@ -32,7 +32,7 @@ public function translate($locale = null, $withFallback = false)
      *
      * @return \Illuminate\Database\Eloquent\Model|null
      */
-    public function translateOrDefault($locale)
+    public function translateOrDefault($locale = null)
     {
         return $this->getTranslation($locale, true);
     }

tests/TranslatableTest.php

@@ -215,6 +215,9 @@ public function test_it_returns_default_translation()
         $this->assertSame($country->getTranslation('ch', true)->name, 'Griechenland');
         $this->assertSame($country->translateOrDefault('ch')->name, 'Griechenland');
         $this->assertSame($country->getTranslation('ch', false), null);
+
+        $this->app->setLocale('ch');
+        $this->assertSame($country->translateOrDefault()->name, 'Griechenland');
     }
 
     public function test_fallback_option_in_config_overrides_models_fallback_option()