added geo centroid aggregation test

tests/Aggregation/GeoCentroidAggregationTest.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\GeoCentroidAggregation;
+
+/**
+ * Unit test for children aggregation.
+ */
+class GeoCentroidAggregationTest extends \PHPUnit_Framework_TestCase
+{
+    /**
+     * Test if exception is thrown when field is not provided
+     *
+     * @expectedException \LogicException
+     */
+    public function testGetArrayException()
+    {
+        $aggregation = new GeoCentroidAggregation('foo');
+        $aggregation->getArray();
+    }
+
+    /**
+     * Tests getType method.
+     */
+    public function testGeoCentroidAggregationGetType()
+    {
+        $aggregation = new GeoCentroidAggregation('foo');
+        $this->assertEquals('geo_centroid', $aggregation->getType());
+    }
+
+    /**
+     * Tests getArray method.
+     */
+    public function testGeoCentroidAggregationGetArray()
+    {
+        $aggregation = new GeoCentroidAggregation('foo', 'location');
+        $this->assertEquals(['field' => 'location'], $aggregation->getArray());
+    }
+}