[2.1.x] Fixed #29400 -- Fixed crash in custom template filters that use decorated functions.

Regression in 620e9dd31a.

Backport of a8d12bc280 from master

tests/template_tests/test_custom.py

@@ -25,6 +25,11 @@ class CustomFilterTests(SimpleTestCase):
             "abcde"
         )
 
+    def test_decorated_filter(self):
+        engine = Engine(libraries=LIBRARIES)
+        t = engine.from_string('{% load custom %}{{ name|make_data_div }}')
+        self.assertEqual(t.render(Context({'name': 'foo'})), '<div data-name="foo"></div>')
+
 
 class TagTestCase(SimpleTestCase):