Fix PathItem::resolveReferences() for array fields

The previous code `$this->$attribute[$k] = $referencedObject` had
no effect  if the value of `$attribute` is an array (as for instance
in the case of `parameters`), because `$this->$attribute` invokes
`SpecBaseObject::__get()`, which does *not* return a reference. Indeed
PHP issued `Notice: Indirect modification of overloaded property
cebe\openapi\spec\PathItem::$parameters has no effect` in this case.

Author: rv1971

src/spec/PathItem.php

@@ -174,7 +174,8 @@ public function resolveReferences(ReferenceContext $context = null)
                     foreach ($this->$attribute as $k => $item) {
                         if ($item instanceof Reference) {
                             $referencedObject = $item->resolve();
-                            $this->$attribute[$k] = $referencedObject;
+                            $this->$attribute =
+                              [ $k => $referencedObject ] + $this->$attribute;
                             if (!$referencedObject instanceof Reference && $referencedObject !== null) {
                                 $referencedObject->resolveReferences();
                             }