Add unit test for as_calendar method

Author: rlskoeser

src/undate/undate.py

@@ -197,7 +197,7 @@ def set_calendar(self, calendar: Union[str, Calendar]):
         to change calendar."""
         if calendar is not None:
             # if not passed as a Calendar instance, do a lookup
-            if not isinstance(calendar, Calendar):
+            if isinstance(calendar, str):
                 # look for calendar by upper-case name
                 try:
                     calendar = Calendar[calendar.upper()]

tests/test_undate.py

@@ -130,6 +130,16 @@ def test_calendar(self):
         with pytest.raises(ValueError, match="Calendar `foobar` is not supported"):
             Undate(848, calendar="foobar")
 
+    def test_as_calendar(self):
+        # changes calendar *without* converting dates
+        assert Undate(1243, 5, 7).as_calendar(Calendar.ISLAMIC) == Undate(
+            1243, 5, 7, calendar=Calendar.ISLAMIC
+        )
+        # should also work with string
+        assert Undate(1243, 5, 7).as_calendar("islamic") == Undate(
+            1243, 5, 7, calendar=Calendar.ISLAMIC
+        )
+
     def test_init_invalid(self):
         with pytest.raises(ValueError):
             Undate("19??")