Add unit tests for middleware

**Feature request**
It would be great to have retry for core.

**Use case**
Many users need this functionality for production deployments.

**Implementation idea**
Add a new config option `enable_retry` with default `false`.

The issue names middleware, core, and the enable_retry configuration option, but provides no files, tests, or entry points. Start by locating the middleware and core configuration, then clarify the retry behavior and how the default false setting should work. Done means the behavior and its unit-test coverage are defined.

Written by the indexing model from the issue text.