The get_property_xxx of Msg class may rise exception

[lihuize123123]

If the property key is missing, the get_property_xxx will rise an exception. The framework user must write try... catch... to deal the exception. If the interface allows default value passed, then the code of framework user will be simpler.

[halajohn]

Do we have any method to distinguish whether the value of a property is the default value when the key is missing, or if the key exists but the value is the default value?