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

[did-vmonroig]

@madorin, I understand my fix is not good but I tried to made progress with no luck. Something is broken in Fibplus when using Delphi from 10.x to most recent. Issues #77 and #83 seems related to this.