fix relationship object

Author: tuyakhov

src/Serializer.php

@@ -111,7 +111,7 @@ protected function serializeModel($model)
                     $relationship = ['id' => $items->getId(), 'type' => $items->getType()];
                 }
                 if (!empty($relationship)) {
-                    $data['relationships'][$name] = $relationship;
+                    $data['relationships'][$name]['data'] = $relationship;
                 }
             }
         }

tests/SerializerTest.php

@@ -87,7 +87,7 @@ public function testExpand()
                 'field2' => 2,
             ],
             'relationships' => [
-                'extraField1' => ['id' => '123', 'type' => 'resource-models']
+                'extraField1' => ['data' => ['id' => '123', 'type' => 'resource-models']]
             ]
         ];
         $model = new ResourceModel();