Handle case when headers argument of ResponseStream constructor is None (#2729)

* Handle case when headers is None

* Add test for response stream with default headers

* Move test

---------

Co-authored-by: Adam Hopkins <adam@amhopkins.com>

sanic/response/types.py

@@ -522,7 +522,9 @@ class ResponseStream:
         headers: Optional[Union[Header, Dict[str, str]]] = None,
         content_type: Optional[str] = None,
     ):
-        if not isinstance(headers, Header):
+        if headers is None:
+            headers = Header()
+        elif not isinstance(headers, Header):
             headers = Header(headers)
         self.streaming_fn = streaming_fn
         self.status = status