fix settings to support falsy valued overrides

Up until recently there were settings with truthy defaults but with no
need to be set to a false value. That changed with
OIDC_INTROSPECTION_VALIDATE_AUDIENCE_SCOPE. Now there is a setting that
has both a true default and a meaningful false value, and without this
fix that setting cannot be changed making it not much of a setting at
all.

oidc_provider/settings.py

@@ -203,7 +203,8 @@ def get(name, import_str=False):
         default_value.update(value)
         value = default_value
     else:
-        value = value or default_value
+        if value is None:
+            value = default_value
         value = import_from_str(value) if import_str else value
 
     return value

oidc_provider/tests/cases/test_settings.py

@@ -23,3 +23,7 @@ class SettingsTest(TestCase):
         key1 = settings.get('OIDC_UNAUTHENTICATED_SESSION_MANAGEMENT_KEY')
         key2 = settings.get('OIDC_UNAUTHENTICATED_SESSION_MANAGEMENT_KEY')
         self.assertEqual(key1, key2)
+
+    @override_settings(OIDC_INTROSPECTION_VALIDATE_AUDIENCE_SCOPE=False)
+    def test_can_override_with_false_value(self):
+        self.assertFalse(settings.get('OIDC_INTROSPECTION_VALIDATE_AUDIENCE_SCOPE'))