Fix return type annotation on get_creation_date()

`_DAVResource.get_creation_date()` is annotated as returning
`Optional[datetime]`, but `_DAVResources.get_property_value()` passes the
method's return value to `get_rfc3339_time()`, which requires a Unix timestamp
instead.

wsgidav/dav_provider.py

@@ -220,7 +220,7 @@ def get_content_type(self) -> Optional[str]:
             return None
         raise NotImplementedError
 
-    def get_creation_date(self) -> Optional[datetime]:
+    def get_creation_date(self) -> Optional[float]:
         """Records the time and date the resource was created.
 
         The creationdate property should be defined on all DAV compliant