Make defaultState non-optional

[johanneslumpe]

Since we have to return a non-undefined state in redux, we should probably make the defaultState parameter for handleActions required. Thoughts?

[lukewestby]

I agree. It would be way more helpful for people to see a clear and explicit error up front than to try and figure out why Redux is complaining about their reducer all the way downstream.

[timche]

+1

I'll probably open a PR for this.

[yangmillstheory]

@timche are you still working on this, or can I self-assign?

[timche]

You can if you want.

[timche]

Fixed in #127.