Allow using non-true values in app kwargs

Trying to instantiate Celery app with non-true kwargs will not work
for those configs which have True as default, for example,
this will not have effect:

>>> app = Celery(task_create_missing_queues=False)
>>> app.conf['task_create_missing_queues']
True

This fix simply changes the filtering which from now on will discard
None values only.

Fixes: celery#6865

Author: alesolda

celery/app/base.py

@@ -323,7 +323,7 @@ def on_init(self):
         """Optional callback called at init."""
 
     def __autoset(self, key, value):
-        if value:
+        if value is not None:
             self._preconf[key] = value
             self._preconf_set_by_auto.add(key)
 

t/unit/app/test_app.py

@@ -274,7 +274,11 @@ def test_with_broker(self, patching):
         with self.Celery(broker='foo://baribaz') as app:
             assert app.conf.broker_url == 'foo://baribaz'
 
-    def test_pending_confugration__kwargs(self):
+    def test_pending_configuration_non_true__kwargs(self):
+        with self.Celery(task_create_missing_queues=False) as app:
+            assert app.conf.task_create_missing_queues is False
+
+    def test_pending_configuration__kwargs(self):
         with self.Celery(foo='bar') as app:
             assert app.conf.foo == 'bar'