Added tests for the Arr::keys() method

tests/Arr/KeysTest.php

@@ -0,0 +1,31 @@
+<?php
+
+namespace Yetii\Tests\Arr;
+
+use PHPUnit\Framework\TestCase;
+use YeTii\General\Arr;
+
+/**
+ * Class KeysTest
+ */
+class KeysTest extends TestCase
+{
+    /**
+     * @var array
+     */
+    public $testData = [
+        'apples',
+        'oranges',
+        'apples',
+    ];
+
+    /**
+     * Test whether it returns an array of the keys
+     */
+    public function testCanGetKeysForArray()
+    {
+        $array = new Arr($this->testData);
+        $result = $array->keys();
+        $this->assertTrue($result === [0, 1, 2]);
+    }
+}