StaticListableBeanFactory.isSingleton() returns false for singleton beans

### Overview

`StaticListableBeanFactory.isSingleton()` returns `false` for all singleton beans other than singletons created by a `FactoryBean`.

## Deliverables

- [x] Ensure `StaticListableBeanFactory.isSingleton()` returns `true` for all beans not created by a `FactoryBean`.