tests/Gateway/post_upload: Fix assert

* dc_msg is a webtest object, so need to explicitly check its `body`.
* And that `body` is bytes, not str, so test against `b'OK'`.

src/tests/gateway/test_post_upload.py

@@ -27,4 +27,4 @@ def test_plain_message(eddn_message: Callable) -> None:
         },
     )
 
-    assert dc_msg == 'OK'
+    assert dc_msg.body == b'OK'