Make sure included is always a plain array so it is serialized as array

Author: JaZo

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 * Omit item from included when it has no attributes or relationships (only type and id)
 N.B. This is a breaking change if you implement the `ItemInterface` yourself instead of using the supplied `Item`.
+* Make sure included is always a plain array so it is serialized as array
 
 ## [0.12.1] - 2019-01-11
 

src/Item.php

@@ -221,11 +221,13 @@ function (ItemInterface $item) use (&$included) {
             }
         }
 
-        return $included->unique(
-            function (array $item) {
-                return $item['type'].':'.$item['id'];
-            }
-        );
+        return $included
+            ->unique(
+                function (array $item) {
+                    return $item['type'].':'.$item['id'];
+                }
+            )
+            ->values();
     }
 
     /**