Fix setting end-of-event false after its true

Author: sfgeorge

lib/sippy_cup/media/dtmf_payload.rb

@@ -24,7 +24,7 @@ def end_of_event=(bool)
         if bool
           @flags |= END_OF_EVENT
         else
-          @flags &= (0xf - END_OF_EVENT)
+          @flags &= ~END_OF_EVENT
         end
       end