Basic auth REDIRECT_HTTP_AUTHORIZATION support

Hi,
in .htaccess, we use the flag `E=HTTP_AUTHORIZATION:%{HTTP:Authorization}` to transfer basic auth credentials to PHP when it is running as CGI/FCGI.
Depending on the Apache configuration, the authorization string is copied either to `$_SERVER['HTTP_AUTHORIZATION']` or to `$_SERVER['REDIRECT_HTTP_AUTHORIZATION']`. We should support this in `$auth->basic`.