Fix inconsistent behavior between setState in init and directly assigning to state

[ZoteTheMighty]

In Roact 1.x, setState is allowed during a component's init method. This change was made to unify all state changes and to further discourage direct mutation of state.

However, getDerivedStateFromProps remains an ever-present source of finicky corner cases. In the case that users were still assigning directly to state, they would actually lose any derived state that was supposed to be present by the time they rendered.

This actually breaks existing components migrating from Roact 0.2.0 to Roact 1.x. This change addresses that incompatibility.

Checklist before submitting:

• Added entry to CHANGELOG.md
• Added/updated relevant tests
• Added/updated documentation