Remove arity check from ref prop type

[ahuth]

In #54 we added a ref prop type, and in #55 added an arity check for callback refs. Unfortunately, that breaks a common pattern of having an empty function as a default value. For example:

function SomeButton({ buttonRef }) {
  return <button buttonRef={buttonRef} />;
}

SomeButton.defaultProps = {
  buttonRef() {}
};

We could leave the arity check and change default values to undefined, instead. However, that's an unknown amount of work, and I think we're better off removing the arity check and accepting functions with any number of arguments.

@ljharb , what do you think?

[ljharb]

Fair point; we could also accept both 0 and 1 but then are we really rejecting anything useful?

[ahuth]

Thanks!