Default value for ownerKey;

Author: hans-thomas

src/Eloquent/HybridRelations.php

@@ -221,7 +221,7 @@ public function morphTo($name = null, $type = null, $id = null, $ownerKey = null
                 $this->newQuery(),
                 $this,
                 $id,
-                $ownerKey,
+                $ownerKey ?: $this->getKeyName(),
                 $type,
                 $name,
             );