Test: Assert that removeClass(undefined) is chainable.

[Krinkle]

Follows-up 227c49a / gh-913.

Assert that the return value is strictEqual to $div. That way it doesn't just assert it being a harmless (e.g. that undefined doesn't mean "remove all classes", but "do nothing"), but also that it is still chainable.

@dmethvin

[Krinkle]

On second thought, it may not be as useful to assert. I was confusing the argument.length logic with methods that are getter or setter depending on that logic (which previously used a false-y or undefined check). However in the case of removeClass this is different since it is always chainable.

[dmethvin]

True, it's not a getter/setter. I think I'll leave it out then.