Fix Pos,Neg contracts which fail incorrectly for nil values

[betamatt]

Pos and Neg contracts explode when passed nil values. This is most easily noticed when combined with Maybe - Maybe[Pos] - which will error when a positive value is not provided.

For example:

  1) Contracts: Pos: should fail for nil
     Failure/Error: expect { @o.pos_test(nil) }.to raise_error(ContractError)
       expected ContractError, got #<NoMethodError: undefined method `>' for nil:NilClass>

[egonSchiele]

Thanks!