added ability to disable TLS channel binding (#1368)

Co-authored-by: Efstratios Goudelis <stratos.goudelis@gmail.com>

autobahn/wamp/auth.py

@@ -158,7 +158,7 @@ def __init__(self, **kw):
                     "Must provide '{}' for cryptosign".format(key)
                 )
         for key in kw.get('authextra', dict()):
-            if key not in ['pubkey']:
+            if key not in ['pubkey', 'channel_binding']:
                 raise ValueError(
                     "Unexpected key '{}' in 'authextra'".format(key)
                 )