check is_numeric instead of is_float for $priority
check is_numeric instead of is_float for $priority

fix https://github.com/silverstripe-labs/silverstripe-googlesitemaps/issues/66#issuecomment-52112157
andrelohmann/silverstripe-googlesitemaps,silverstripe-rebelalliance/silverstripe-googlesitemaps,wilr/silverstripe-googlesitemaps,silverstripe-labs/silverstripe-googlesitemaps