Callable checks for query builder causes trouble with built-in Laravel functions

[Radiergummi]

We discovered that when trying to work with index fields that have the same name as global functions, PHP will report them as callable, and the library will treat ordinary field names as custom condition callbacks. This messes up query building and makes it impossible to use fields such as url.
The check should be narrowed to Closures to prevent those issues from cropping up.