Date format in validation callback overrides the date format in elements that extend localgov_date element

[andybroomfield]

With the fix in #72 which provides a validation date format, there is a side effect that when an element that extends localgov_forms_date provides a custom date format, this is overidden by the validation method.

We noticed that you can get the validation method set correctly in the element decleration in the getInfo() decleration, setting date_date_format to the required format.

[MattOz-CDS]

Hey @andybroomfield , thanks for highlighting this side effect. I'll follow your recommendation in order to fix it..