Fix TFIBCustomDataSet.DoFieldValidate getting empty value on date fields

[did-vmonroig]

Previous fix #76 broke validation of date fields, entering on their OnValidate event always with Null value.

[madorin]

@did-vmonroig , can you make a simple test case with two fields date and integer or other types to investigate it better?
I see TFIBDateField.SetAsDateTime have a strange override and it seems need to be fixed in another way.

Unfortunatelly I cannot accept this case specific merge request with if Field is TIntegerField then