added a test

tests/Aggregation/Pipeline/SerialDifferencingAggregationTest.php

+<?php
+
+/*
+ * This file is part of the ONGR package.
+ *
+ * (c) NFQ Technologies UAB <info@nfq.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace ONGR\ElasticsearchDSL\Tests\Aggregation\Pipeline;
+
+use ONGR\ElasticsearchDSL\Aggregation\Pipeline\SerialDifferencingAggregation;
+
+/**
+ * Unit test for serial differencing aggregation.
+ */
+class SerialDifferencingAggregationTest extends \PHPUnit_Framework_TestCase
+{
+    /**
+     * Tests toArray method.
+     */
+    public function testToArray()
+    {
+        $aggregation = new SerialDifferencingAggregation('acme', 'test');
+        $aggregation->addParameter('lag', '7');
+
+        $expected = [
+            'serial_diff' => [
+                'buckets_path' => 'test',
+                'lag' => '7'
+            ],
+        ];
+
+        $this->assertEquals($expected, $aggregation->toArray());
+    }
+}