[4.2] Some substitutions of typehint (helpers.php)

No way!!

As [I've said](https://github.com/laravel/framework/pull/4828#issuecomment-47393875) many times [before](https://github.com/laravel/framework/pull/4276#issuecomment-45563986) (but nobody seems to listen), this is extremely dangerous.

If you accept a non-callable string, you ***cannot*** use `is_callable`. You *must* check for a `Closure` directly.

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This is true in other parts of your code here, as well as some of your changes in #5045.

Please be extra vigilant about these. Thank you.