Fix casting

Author: Mauko Quiroga

openfisca_core/periods/instant_.py

@@ -199,7 +199,10 @@ def period(self, unit: DateUnit, size: int = 1) -> Timeable:
             f"Invalid size: {size} of type {type(size)}. Expecting any " \
             "int >= 1."
 
-        return periods.period(f"{DateUnit[unit].value}:{str(self)}:{size}")
+        if isinstance(unit, str):
+            unit = DateUnit[unit]
+
+        return periods.period(f"{unit.value}:{str(self)}:{size}")
 
     @functools.lru_cache(maxsize = None)
     def offset(self, offset: OffsetBy, unit: DateUnit) -> Instant: