Fix Daphne ASGI compatibility and document WS handshake cookie limitation

- AsgiContext and AsgiWebSocket now default missing  scope key
  to 'http' / 'ws' respectively (Daphne omits it for websocket scopes)
- Add regression test for websocket scope without scheme
- Update example/session.py WS handler to read the session; cookie must
  be set via HTTP first because common ASGI servers ignore the headers
  field on websocket.accept
- Update README with the same caveat about Granian/Daphne/curl

example/session.py

@@ -35,7 +35,11 @@ async def echo(ws: WebSocket) -> None:
 
 @app.websocket('/ws/visits')
 async def ws_visits(ws: WebSocket) -> None:
-    visits = ws.session.get('visits', 0) + 1
-    ws.session['visits'] = visits
+    # WebSocket handlers can read the existing session. Most ASGI servers
+    # (including Granian and Daphne) do not forward the `headers` field of the
+    # `websocket.accept` message into the HTTP 101 response, so a new session
+    # cookie cannot be set during the handshake. Use the HTTP `/` endpoint to
+    # set or refresh the session cookie before connecting here.
     await ws.accept()
+    visits = ws.session.get('visits', 0)
     await ws.send_text(f'visits: {visits}')