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

tests/test_response.py

@@ -23,6 +23,7 @@ from sanic.compat import Header
 from sanic.cookies import CookieJar
 from sanic.response import (
     HTTPResponse,
+    ResponseStream,
     empty,
     file,
     file_stream,
@@ -943,3 +944,17 @@ def test_file_validating_304_response(
     )
     assert response.status == 304
     assert response.body == b""
+
+
+def test_stream_response_with_default_headers(app: Sanic):
+    async def sample_streaming_fn(response_):
+        await response_.write("foo")
+
+    @app.route("/")
+    async def test(request: Request):
+        return ResponseStream(sample_streaming_fn, content_type="text/csv")
+
+    _, response = app.test_client.get("/")
+    assert response.text == "foo"
+    assert response.headers["Transfer-Encoding"] == "chunked"
+    assert response.headers["Content-Type"] == "text/csv"