Fixed TestClient error when response headers missing (#1200)
Fixed TestClient error when response headers missing (#1200)

* Fixed https://github.com/abersheeran/asgi-ratelimit/issues/14

* lint it

* Black it

Co-authored-by: euri10 <52f19337d2d29bb686068057076a80000a23296b@users.noreply.github.com>
encode/starlette,encode/starlette