Use non-nullable parameters for operator ==

[natebosch]

The null value will never be passed to the equals operator
implementation. Widening the type causes any other class which
implements this interface to also widen the type.

[coveralls]

Coverage remained the same at 49.489% when pulling 30455fd on non-null-equals-parameter into 1c72944 on master.

[kevmoo]

It might be worth adding a change on entry here. Are there other planned changes or should you just release now?

[natebosch]

This isn't very user visible, but I can add a changelog.

I'm eagerly cleaning this up because I hope we can add a lint.

[natebosch]

Are there other planned changes or should you just release now?

No other changes planned. It's not terribly important to publish IMO, but I can drop the -dev and we can publish after internal validation.