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_reqctx.py

@@ -48,6 +48,21 @@ def test_teardown_with_previous_exception():
         assert buffer == []
     assert buffer == [None]
 
+def test_teardown_with_handled_exception():
+    buffer = []
+    app = flask.Flask(__name__)
+    @app.teardown_request
+    def end_of_request(exception):
+        buffer.append(exception)
+
+    with app.test_request_context():
+        assert buffer == []
+        try:
+            raise Exception('dummy')
+        except Exception:
+            pass
+    assert buffer == [None]
+
 def test_proper_test_request_context():
     app = flask.Flask(__name__)
     app.config.update(