Add an object.append() method

[kanongil]

While the object.keys(schema) method allows an existing schema to be extended, the behaviour when no keys are present does not align with what I expect. Namely undefined, null, & {} does not extend the existing schema, but for some reason replaces it.

This means that if I want to optionally extend a schema, I will need to manually test the extension for these values, to avoid calling object.keys() with it.

As such, I would like to propose an object.append(schema) method, which will do nothing for these values, and otherwise do what object.keys(schema) does.

[Marsup]

I guess there's no harm in having a utility method skipping this, fwiw those behaviors are well documented.
I'll take a PR.