Consider CaseInsensitiveArray an associative array.

tests/PHPCurlClass/ArrayUtilTest.php

@@ -0,0 +1,15 @@
+<?php
+
+namespace CurlTest;
+
+use Curl\ArrayUtil;
+use Curl\CaseInsensitiveArray;
+
+class ArrayUtilTest extends \PHPUnit\Framework\TestCase
+{
+    public function testCaseInsensitiveArrayIsArrayAssoc()
+    {
+        $array = new CaseInsensitiveArray();
+        $this->assertTrue(ArrayUtil::isArrayAssoc($array));
+    }
+}