Fix registration of async functions (#1201)

* bug fix for async functions

* Update test_conversable_agent.py

Co-authored-by: Chi Wang <wang.chi@microsoft.com>

* Update test/agentchat/test_conversable_agent.py

Co-authored-by: Chi Wang <wang.chi@microsoft.com>

* commented out cell in a notebook until issue #1205 is not fixed

---------

Co-authored-by: Chi Wang <wang.chi@microsoft.com>