Switch away from using None as default value for the exception when tearing down a context.

When an exception has been handled when using the request / app context in a with statement, `sys.exc_info()` will still contain the exception information even though it has been handled already. The `__exit__` methods pass in `None` for the exception value in that case, which needs to be distinguisable from the default value for the `exc` parameter. Use a dedicated singleton sentinel value instead.

tests/test_appctx.py

@@ -78,6 +78,21 @@ def test_app_tearing_down_with_previous_exception():
 
     assert cleanup_stuff == [None]
 
+def test_app_tearing_down_with_handled_exception():
+    cleanup_stuff = []
+    app = flask.Flask(__name__)
+    @app.teardown_appcontext
+    def cleanup(exception):
+        cleanup_stuff.append(exception)
+
+    with app.app_context():
+        try:
+            raise Exception('dummy')
+        except Exception:
+            pass
+
+    assert cleanup_stuff == [None]
+
 def test_custom_app_ctx_globals_class():
     class CustomRequestGlobals(object):
         def __init__(self):