Use phpunit isString instead of defining it.

Author: GhaziTriki

tests/TestCase.php

@@ -289,14 +289,6 @@ protected function minifyString($string)
 
     // Additional assertions
 
-    public function assertIsString($actual, $message = '')
-    {
-        if (empty($message)) {
-            $message = 'Got a ' . gettype($actual) . ' instead of a string';
-        }
-        $this->assertTrue(is_string($actual), $message);
-    }
-
     public function assertIsInteger($actual, $message = '')
     {
         if (empty($message)) {