Update user_client.py

Fixes #77

No, still getting the error
06aa6e2 (file mk2/user_client.py, line 415)
Error:
self._default = list(set(self.HIDE, self.SHOW) - set(action))[0]
exceptions.TypeError: set expected at most 1 arguments, got 2

Why not using this :
 
    if action == self.HIDE:
      self._default = self.SHOW
    else:
      self._default = self.HIDE