change default uri_protocol configruation

The default setting of 'AUTO'  doesn't work well on Apache with rewrite_mod.
There is a problem using "PATH_INFO" to _set _uri_string.
https://github.com/EllisLab/CodeIgniter/blob/develop/system/core/URI.php#L106

The default setting of 'AUTO'  works well on CI 2.1.4, because it checks 'REQUEST_URI' first:
https://github.com/EllisLab/CodeIgniter/blob/2.1.4/system/core/URI.php#L97

And I find that there is a simple question on stackoverflow:
http://stackoverflow.com/questions/12483659/htaccess-broke-serverpath-info