Fix bug with Date Apply

[LeComptoirDesPharmacies]

According to #9 , there is a bug when applying JSON patch with fields as "Date".
These should be custom managed like Arrays.

I did not use function to test Date from external library (as lodash for example) in order to minimize library size.
Some tests were added to test these specific cases.

[pierreinglebert]

Thanks for the PR,
As I just wrote in the issue #9 Date representation is not specified in JSON so it's kinda an expected behavior. I'm looking for the possibility to apply toJSON() (will work for date).

[LeComptoirDesPharmacies]

Oh okay, I understand your solution of using toJSON for these objects. As Date use ISO 8601 representation it is a good idea. What could be strange is that Date object is converted to String on apply / generate but it may be fine for some usages. ;)