Modified extend to take into account primitive and absent values

[delapuente]

Fix #2809

With the addition of the extend() function, the extension mechanism can consider the cases in which the component is absent or is of a primitive type:

1. If both base and extension are absent, extend() will return undefined.
2. Else, If base or extension is absent, extend() will returns a copy of the other.
3. Else, If none of them is absent and both are pure JS objects, the second will extend the first as usual.
4. Finally, the extension is preferred so a copy of it is returned.

[ngokevin]

Can you post example code which this PR might address? Will help clarify the case, thanks!

[dmarcos]

I would add a test with the case that this PR fixes

[ngokevin]

added the tests