[fix] calendar field

I think a strange behavior was introduced in https://github.com/akeeba/fof/pull/289

After that PR the default value is forced to 'now' or 0 which I think is wrong. In my opinion we should allow the field to be shown empty. Neither `1970-01-01` or `-0001-11-30` are a good value to show. 

This PR allows you to use the default value you want. For example if you really like to see a 1970 date:

```xml
		<field name="due" type="calendar"
			class="inputbox"
			label="COM_TODO_ITEMS_FIELD_DUE"
			labelclass="todo-label"
			required="true"
			size="20"
			default="1970-01-01"
		 />
```

The value 'now' still works as expected because anything passed will be directly passed to the date object.

Summary:
* Empty default value = no date shown 
* `now` = current date
* Any other default value = JDate is initialised with that value