Avoid KeyError when no room exists

[pdecat]

Without this modification, base_manager.get_rooms() fails with a KeyError when no room exists:

ERROR: test_no_room (tests.test_base_manager.TestBaseManager)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/pdecat/workspaces/thirdparty/python-socketio/tests/test_base_manager.py", line 135, in test_no_room
    rooms = self.bm.get_rooms('123', '/foo')
  File "/home/pdecat/workspaces/thirdparty/python-socketio/socketio/base_manager.py", line 86, in get_rooms
    for room_name, room in six.iteritems(self.rooms[namespace]):
KeyError: '/foo'

[miguelgrinberg]

Thanks!