Only call _clear_headers_for_304 on 304 responses

Fixes #2726.

Signed-off-by: Anders Kaseorg <[email protected]>

tornado/web.py

@@ -1138,10 +1138,11 @@ def finish(self, chunk: Optional[Union[str, bytes, dict]] = None) -> "Future[Non
                 assert not self._write_buffer, (
                     "Cannot send body with %s" % self._status_code
                 )
-                self._clear_headers_for_304()
             elif "Content-Length" not in self._headers:
                 content_length = sum(len(part) for part in self._write_buffer)
                 self.set_header("Content-Length", content_length)
+            if self._status_code == 304:
+                self._clear_headers_for_304()
 
         assert self.request.connection is not None
         # Now that the request is finished, clear the callback we