Solve for React.Fragment being returned from rehydrateChildren

[simon360]

rehydrateChildren will return one of two results:

1. a single node, if an element only contains a single child
2. a React.Fragment, if an element contains > 1 child

Wrapping children in a React.Fragment can interfere with React.Children.forEach (and the other React.children functions).

The previous behaviour in wel-ui was to return an array. Perhaps it's worth reverting to that, and always returning an array?

[simon360]

Current thinking:

• rehydrateChildren can return either a React.Fragment, or a ReactNode. Users of it should be able to handle either.
• Because of this behaviour, it needs clear documentation in the rehydrateChildren docs.