Fix Django 1.0 middleware function compatibility

Author: ryankask

debug_toolbar/apps.py

@@ -1,5 +1,7 @@
 from __future__ import absolute_import, unicode_literals
 
+import inspect
+
 from django.apps import AppConfig
 from django.conf import settings
 from django.core.checks import Error, register
@@ -63,4 +65,7 @@ def is_middleware_class(middleware_class, middleware_path):
         middleware_cls = import_string(middleware_path)
     except ImportError:
         return
-    return issubclass(middleware_cls, middleware_class)
+    return (
+        inspect.isclass(middleware_cls) and
+        issubclass(middleware_cls, middleware_class)
+    )

tests/middleware.py

@@ -0,0 +1,7 @@
+def simple_middleware(get_response):
+    """
+    Used to test Django 1.10 compatibility.
+    """
+    def middleware(request):
+        return get_response(request)
+    return middleware

tests/test_integration.py

@@ -243,3 +243,15 @@ def test_check_gzip_middleware_error(self):
                 ),
             ]
         )
+
+    @override_settings(
+        MIDDLEWARE=[
+            'debug_toolbar.middleware.DebugToolbarMiddleware',
+            'tests.middleware.simple_middleware',
+        ],
+        MIDDLEWARE_CLASSES=None
+    )
+    def test_middleware_factory_functions_supported(self):
+        messages = run_checks()
+        self.assertEqual(len(messages), 1)
+        self.assertEqual(messages[0].id, '1_10.W001')