Fix recurrence rule UNTIL string serialization (#122)

* Update recurrence rule marshalling

* Use type instead of key for value serialization

ical/types/recur.py

@@ -286,6 +286,8 @@ def __encode_property_value__(cls, data: dict[str, Any]) -> str:
                         occurrence = ""
                     values.append(f"{occurrence}{weekday}")
                 value = ",".join(values)
+            elif isinstance(value, datetime.datetime):
+                value = DateTimeEncoder.__encode_property_json__(value)
             if not value:
                 continue
             result.append(f"{key.upper()}={value}")

tests/types/test_recur.py

@@ -669,3 +669,18 @@ def test_recur_as_string() -> None:
     )
     assert event.rrule
     assert event.rrule.as_rrule_str() == "FREQ=DAILY;INTERVAL=2"
+
+
+def test_recur_until_as_string() -> None:
+    """Test converting a recurrence rule with a date into a string."""
+
+    event = Event(
+        summary="summary",
+        start=datetime.datetime(2022, 8, 1, 6, 0, 0),
+        end=datetime.datetime(2022, 8, 2, 6, 30, 0),
+        rrule=Recur(
+            freq=Frequency.DAILY, until=datetime.datetime(2022, 8, 10, 6, 0, 0)
+        ),
+    )
+    assert event.rrule
+    assert event.rrule.as_rrule_str() == "FREQ=DAILY;UNTIL=20220810T060000;INTERVAL=1"