added a test for parent inner hit

tests/InnerHit/ParentInnerHitTest.php

+<?php
+
+namespace ONGR\ElasticsearchDSL\Tests\InnerHit;
+
+use ONGR\ElasticsearchDSL\InnerHit\ParentInnerHit;
+use ONGR\ElasticsearchDSL\Query\TermQuery;
+
+class ParentInnerHitTest extends \PHPUnit_Framework_TestCase
+{
+    public function testToArray()
+    {
+        $query = new TermQuery('foo', 'bar');
+        $hit = new ParentInnerHit('test', 'acme', $query);
+        $expected = [
+            'type' => [
+                'acme' => [
+                    'query' => $query->toArray(),
+                ],
+            ],
+        ];
+        $this->assertEquals($expected, $hit->toArray());
+    }
+}