BaseResponse: ItemList must be called by process() method for hydrate…

… all object types.

src/Response/BaseResponse.php

@@ -168,7 +168,7 @@ private function process($haystack, array $trackedInstanceHashes = [])
 				];
 			}
 			if ($haystack instanceof ItemsList) {
-				return $haystack->getData();
+				return $this->process($haystack->getData(), $trackedInstanceHashes);
 			}
 			if (\method_exists($haystack, '__toString') === true) {
 				return (string) $haystack;