Simplify ZPP code

[twose]

No description provided.

[TysonAndre]

array_is_list(array): bool was proposed in php#4886 (comment) , but I'm opposed to doing that - is_numeric() doesn't throw for objects, for example. This is implementing is_list(mixed): bool, and should not throw for non-array types - it's already as short as is practical

This is why the tests fail with your proposed change.