Convert stub nil arguments to NSNull before adding to params array

Stubbing a method that is called with nil arguments currently fails because [KWStub attempts to add nil to an array](https://github.com/allending/Kiwi/blob/master/Classes/KWStub.m#L184-L188). 

The following method:

    - (void)someMethod
    {
        [otherObject someOtherMethodWithNilArgument:nil];
    }

stubbed using:

    [otherObjectMock stub:@selector(someMethod) withBlock:^id(NSArray *theArguments) {
        // Do something   
    }

will never call the block because an attempt is made to add nil to an array which raises an exception which is [caught](https://github.com/allending/Kiwi/blob/master/Classes/KWMock.m#L485) by `KWMock`. 

This PR fixes the issue by inserting `NSNull` in stead of nil into the array.