Add test case for unsupported serialization

Author: rlskoeser

src/undate/converters/combined.py

@@ -82,4 +82,4 @@ def parse(self, value: str) -> Union[Undate, UndateInterval]:
 
     def to_string(self, undate: Union[Undate, UndateInterval]) -> str:
         "Not supported by this converter. Will raise :class:`ValueError`"
-        raise ValueError("Omnibus supporter does not support serialization")
+        raise ValueError("Omnibus converter does not support serialization")

tests/test_converters/test_combined_parser.py

@@ -38,3 +38,8 @@ def test_transform(date_string, expected):
 def test_converter(date_string, expected):
     # should work the same way when called through the converter class
     assert repr(Undate.parse(date_string, "omnibus")) == repr(expected)
+
+
+def test_no_serialize():
+    with pytest.raises(ValueError, match="does not support"):
+        Undate("2022").format("omnibus")