Advertise support for client_max_window_bits by default.

This doesn't change anything in the default setup (compression="deflate")
because it sets client_max_window_bits to 12 explicitly.

If a client is configured with extensions=[ClientPerMessageDeflateFactory()]
this change gives the server the option to limit client_max_window_bits.

Specifically, this makes such a client compatible with a websockets server
using the default setup (compression="deflate").

Author: aaugustin

src/websockets/extensions/permessage_deflate.py

@@ -286,7 +286,7 @@ def __init__(
         server_no_context_takeover: bool = False,
         client_no_context_takeover: bool = False,
         server_max_window_bits: Optional[int] = None,
-        client_max_window_bits: Optional[Union[int, bool]] = None,
+        client_max_window_bits: Optional[Union[int, bool]] = True,
         compress_settings: Optional[Dict[str, Any]] = None,
     ) -> None:
         """

tests/legacy/test_client_server.py

@@ -834,7 +834,12 @@ def test_extension_client_rejection(self):
             ServerPerMessageDeflateFactory(),
         ]
     )
-    @with_client("/extensions", extensions=[ClientPerMessageDeflateFactory()])
+    @with_client(
+        "/extensions",
+        extensions=[
+            ClientPerMessageDeflateFactory(client_max_window_bits=False),
+        ],
+    )
     def test_extension_no_match_then_match(self):
         # The order requested by the client has priority.
         server_extensions = self.loop.run_until_complete(self.client.recv())