Actions named 'fire' break the scheduler

[olaan]

Using the name 'fire' for an action gives strange results.

2017-05-16 14:12:41,095 ERROR    13136-calvin.calvin.runtime.north.scheduler: unsupported operand type(s) for |=: 'bool' and 'tuple' 
Traceback (most recent call last):                                                                                                   
  File "/home/user/calvin-base/calvin/runtime/north/scheduler.py", line 124, in fire_actors                              
    did_fire |= actor.fire()