Fixed: string representation for "not in" filter comparator should show array

[kcarrasco-panw]

This PR fixes the following bug:
Given the following filter:

f := elemental.NewFilter().WithKey("name").NotIn("abc001", "abc002").Done()

The f.String() representation only shows the first element:

(name not in "abc001")

The correct, expected representation is:

(name not in ["abc001", "abc002"])

Which is analogous to the behaviour of the In comparator:

f := elemental.NewFilter().WithKey("name").In("abc001", "abc002").Done()

which returns the following f.String() representation:

(name in ["abc001", "abc002"])