* use `to_represenation` in favor of `get_attribute`
* fix datetime type does get converted to a string
to_representation will convert the datetime field into a string representation. However the to_representation on the method field will only call its underlying method.
* fix add missing import
* apply black formatter
* add test for serializer method field
* apply black format
* improve backward compatibility
by using date's class contructor instead of fromisostring
* apply black format
* fix black format issue