`replace(…, '_')` is considered as a callable

[Hywan]

Ustring::replace supports 2 forms for the second argument: Either a string, or a callable. If it is a string, then preg_replace is used, else preg_replace_callback.

The string '_' is considered as a callable, because is_callable('_') returns true. Why? Because the function _ exists!

It is a bug. It's better to test if it is a Closure.

cc @hoaproject/hoackers, any volunteers?