Remove notice when request is made and test flag is not set.

Message:
 PHP Notice:  Undefined index: PHP_CURL_CLASS_TEST_MODE_ENABLED in
 ./php-curl-class/tests/PHPCurlClass/server.php on line 4

Author: zachborboa

tests/PHPCurlClass/server.php

@@ -1,7 +1,7 @@
 <?php
 // Prevent direct access unless testing.
 if (getenv('PHP_CURL_CLASS_TEST_MODE_ENABLED') !== 'yes' &&
-    $_SERVER['PHP_CURL_CLASS_TEST_MODE_ENABLED'] !== 'yes') {
+    @$_SERVER['PHP_CURL_CLASS_TEST_MODE_ENABLED'] !== 'yes') {
     exit;
 }