#1 date input field : set_default works differently

[Dieter Adriaenssens]

For date input fields, the set_default function behaves differently than the other input fields.
Calling set_default without parameter changes the value of the field to the default value. If the parameter has a value, the default-value is changed to this value.
For the date_input_field, set_default with a parameter sets the value to now(), ignoring the default value of the date_field.

A similar problem might exist with the time and datetime input fields (untested).