Do not call autoloader when checking if CurlFile class exists

As CurlFile is a built-in class there is no reason to invoke the autoloader, doing so can cause issues for users who define their own autoloader by creating an "__autoload" function or by registering an autoloader with "spl_autoload_register".

I tested this change on Php 5.5 (CurlFile present) and Php 5.3 (CurlFile not present)