Fixed TestClient error when response headers missing (#1200)

* Fixed abersheeran/asgi-ratelimit#14

* lint it

* Black it

Co-authored-by: euri10 <euri10@users.noreply.github.com>

starlette/testclient.py

@@ -214,7 +214,8 @@ async def send(message: Message) -> None:
                 raw_kwargs["status"] = message["status"]
                 raw_kwargs["reason"] = _get_reason_phrase(message["status"])
                 raw_kwargs["headers"] = [
-                    (key.decode(), value.decode()) for key, value in message["headers"]
+                    (key.decode(), value.decode())
+                    for key, value in message.get("headers", [])
                 ]
                 raw_kwargs["preload_content"] = False
                 raw_kwargs["original_response"] = _MockOriginalResponse(