Fix bug "Class 'string' not found"

It's useful to execute isBuiltIntType() function first to avoid built-in type autoloading. Because errors occurs if there are more then one autoloader in autoloading stack and one of them rises an exception if class not found  (for instance, if you use composer autoloader for PayPal libs and framework built-in autoloader for everything else).

Looks like issue https://github.com/paypal/sdk-core-php/issues/7 caused by this bug.