Fix mute_signals behavior for signals with caching

Connecting signals (with use_caching=True) inside mute_signals
was breaking unmute on exit. Paused receivers were not running.
This was caused by signal cache not being restored after unpatching.
Workaround is to clear signal cache on exit.

Fixes FactoryBoy#212

factory/django.py

@@ -277,6 +277,8 @@ def __exit__(self, exc_type, exc_value, traceback):
                          receivers)
 
             signal.receivers = receivers
+            with signal.lock:
+                signal.sender_receivers_cache.clear()
         self.paused = {}
 
     def copy(self):

tests/test_django.py

@@ -678,6 +678,19 @@ def test_context_manager(self):
 
         self.assertSignalsReactivated()
 
+    def test_signal_cache(self):
+        with factory.django.mute_signals(signals.pre_save, signals.post_save):
+            signals.post_save.connect(self.handlers.mute_block_receiver)
+            WithSignalsFactory()
+
+        self.assertTrue(self.handlers.mute_block_receiver.call_count, 1)
+        self.assertEqual(self.handlers.pre_init.call_count, 1)
+        self.assertFalse(self.handlers.pre_save.called)
+        self.assertFalse(self.handlers.post_save.called)
+
+        self.assertSignalsReactivated()
+        self.assertTrue(self.handlers.mute_block_receiver.call_count, 1)
+
     def test_class_decorator(self):
         @factory.django.mute_signals(signals.pre_save, signals.post_save)
         class WithSignalsDecoratedFactory(factory.django.DjangoModelFactory):