Change return value to more concrete object and reinstate override
Change return value to more concrete object and reinstate override

Changed the return value of the omitNullValues method from AbstractToStringHelper to ToStringHelper (enforced unnecessary polymorphism in its previous state) This change reverts one from a previous commit (f8e9926).

Reinstated override of the add method in order to change the return value from AbstractToStringHelper to ToStringHelper.

OliverAbdulrahim/ToStringHelper,OliverAbdulrahim/ToStringHelper