Ignore writing headers when in ASGI mode (#1957)

* Ignore writing headers when in ASGI mode for streaming responses

* Move asgi set on streaming until after response type check

* Adds multidict==5.0.0 to pass tests

* Bump version to 20.9.1

examples/run_asgi.py

@@ -7,8 +7,8 @@
 """
 
 from pathlib import Path
-from sanic import Sanic, response
 
+from sanic import Sanic, response
 
 app = Sanic(__name__)
 
@@ -42,9 +42,7 @@ async def handler_file(request):
 
 @app.route("/file_stream")
 async def handler_file_stream(request):
-    return await response.file_stream(
-        Path("../") / "setup.py", chunk_size=1024
-    )
+    return await response.file_stream(Path("../") / "setup.py", chunk_size=1024)
 
 
 @app.route("/stream", stream=True)