text: Title: Mangum 0.12 regression: Unable to determine handler from trigger event
  Body: A lambda invoke call against mangum 0.11 works fine but with upgrade to version 0.12 same call fails with:
  
  [ERROR] TypeError: Unable to determine handler from trigger event
  Traceback (most recent call last):
    File "/var/task/mangum/adapter.py", line 86, in __call__
      handler = AbstractHandler.from_trigger(
    File "/var/task/mangum/handlers/abstract_handler.py", line 113, in from_trigger
      raise TypeError("Unable to determine handler from trigger event")
  START RequestId: 7d86054b-94df-4bba-b5e0-60106f2d9fa9 Version: $LATEST
  