RyuApp: allow observing events without having handlers

this will be used by ofctl app which registers handlers dynamically.
when unobserving an event cls dynamically, there might be events of
the cls left in app's event queue already.

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>

ryu/base/app_manager.py

@@ -191,6 +191,9 @@ class RyuApp(object):
             return handlers
 
         def test(h):
+            if not ev_cls in h.callers:
+                # this handler does not listen the event.
+                return False
             states = h.callers[ev_cls].dispatchers
             if not states:
                 # empty states means all states