Change the default value type handling

A field property has to be typed.
The type has to be defined based on the following logic:

• get the type defined by the field definition (property type)
• if not defined, get the type defined by the field factory (A date field factory may define a default type : Date)
• if not defined, get the type of the already stored property value (if existing)
• if not existing set the default type (String).