Also, Object.assign mutates the first parameter, which might lead to unexpected side effects. This PR fixes that, too.

@THEtheChad since you've already created the PR I thought about, may I ask you to expand it a little bit? For the sake of demonstrating default parameters, I suggest to remove the fields title and buttonText from menuConfig completely, and change body and cancellable values to something different from default values. Because even with your fix, the first two will become null, and the last two remain unchanged.
I doubt that null values were intentional.

This is a duplicate of #9

Thanks again for spotting the issue here, this really helped put together this fix below. Let me know if you see other issues!
d00ca4c