added significant terms aggregation test

tests/Aggregation/SignificantTermsAggregationTest.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;
+
+use ONGR\ElasticsearchDSL\Aggregation\SignificantTermsAggregation;
+
+/**
+ * Unit test for children aggregation.
+ */
+class SignificantTermsAggregationTest extends \PHPUnit_Framework_TestCase
+{
+    /**
+     * Tests getType method.
+     */
+    public function testSignificantTermsAggregationGetType()
+    {
+        $aggregation = new SignificantTermsAggregation('foo');
+        $result = $aggregation->getType();
+        $this->assertEquals('significant_terms', $result);
+    }
+
+    /**
+     * Tests getArray method.
+     */
+    public function testSignificantTermsAggregationGetArray()
+    {
+        $mock = $this->getMockBuilder('ONGR\ElasticsearchDSL\Aggregation\AbstractAggregation')
+            ->disableOriginalConstructor()
+            ->getMockForAbstractClass();
+        $aggregation = new SignificantTermsAggregation('foo', 'title');
+        $aggregation->addAggregation($mock);
+        $result = $aggregation->getArray();
+        $expected = ['field' => 'title'];
+        $this->assertEquals($expected, $result);
+    }
+}