Only use add method when the value is not ellipsis

Author: nmandery

stac_fastapi/extensions/stac_fastapi/extensions/core/fields/request.py

@@ -34,7 +34,8 @@ def _get_field_dict(fields: Optional[Set[str]]) -> Dict:
                 if parent not in field_dict:
                     field_dict[parent] = {key}
                 else:
-                    field_dict[parent].add(key)
+                    if field_dict[parent] is not ...:
+                        field_dict[parent].add(key)
             else:
                 field_dict[field] = ...  # type:ignore
         return field_dict