Raise ValueError when setting StreamResponse.last_modified to an unsu…

…pported type

Currently this would silently fail and discard the new
value.

fixes #10143

aiohttp/web_response.py

@@ -267,6 +267,9 @@ def last_modified(
             )
         elif isinstance(value, str):
             self._headers[hdrs.LAST_MODIFIED] = value
+        else:
+            msg = f"Unsupported type for last_modified: {type(value).__name__}"
+            raise ValueError(msg)
 
     @property
     def etag(self) -> Optional[ETag]:

tests/test_web_response.py

@@ -242,6 +242,13 @@ def test_last_modified_reset() -> None:
     assert resp.last_modified is None
 
 
+def test_last_modified_invalid_type() -> None:
+    resp = web.StreamResponse()
+
+    with pytest.raises(ValueError, match="Unsupported type for last_modified: object"):
+        resp.last_modified = object()  # type: ignore[assignment]
+
+
 @pytest.mark.parametrize(
     "header_val",
     (