getValue() for MonthSelect, DateSelect and DateTimeSelect Form Elements

[richardjennings]

Tests added to check it is possible to get the value set on a DateTime,Date or Month Select Form Element.

getValue() methods implemented for each DateTime,Date and Month Select Form Elements using sprintf to return the designated format.

Fixes Issue #4755

[tca3]

I reckon they should return DateTime objects, which would make more sense IMHO.

[richardjennings]

@ThomasCantonnet
The return format is specified already as a filter callback in getInputSpecification()

For example $form->getData() will return 'year-month-day' for a DateSelect Element ( here )

The getValue() implementations added return a format identical to $form->getData();

[weierophinney]

Merged to develop for release with 2.3.0, as it's a slight behavior change, and I want folks to test this a bit before we offer it in a release.