Resolve "Problems with dates which are in the new year, but in the last calendar week of the old year"

Closes #5 (closed)

calendarweek/calendarweek.py

@@ -64,7 +64,13 @@ class CalendarWeek:
         """ Get the calendar week by a date object (the week this date is in). """
 
         week = int(when.strftime("%V"))
-        year = when.year + 1 if when.month == 12 and week == 1 else when.year
+        year = when.year
+
+        if when.month == 12 and week == 1:
+            year += 1
+
+        if (when.month == 52 or when.month == 53) and when.month == 1:
+            year -= 1
 
         return cls(year=year, week=week)