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

packages/kaya-core/tests/test_websocket.py

@@ -131,3 +131,23 @@ class WebSocketTest(unittest.TestCase):
         await ws.accept()
         self.assertEqual(1, len(sent_messages))
         self.assertEqual({'type': 'websocket.accept'}, sent_messages[0])
+
+    @async_test
+    async def test_websocket_scope_without_scheme(self):
+        # Daphne omits the optional `scheme` key from websocket scopes.
+        async def send(message):
+            pass
+
+        async def receive():
+            return {'type': 'websocket.connect'}
+
+        scope = {
+            'type': 'websocket',
+            'path': '/echo',
+            'query_string': b'',
+            'client': ('127.0.0.1', 12345),
+            'server': ('127.0.0.1', 80),
+            'headers': [],
+        }
+        ws = AsgiWebSocket(scope, receive, send)
+        self.assertEqual('ws', ws.scheme)