Fix bug where string would cause invariant error.

[eightypop]

This PR fixes a bug that the switch to React.Children exposed.

Children were always either being wrapped or cloned due to the fact that the props object existed even when there were no keys. Before using React.Children to check length for wrapping, we were using props.children.length which when it was a string, it's length property would force it to be wrapped. UsingReact.Children.count no longer allowed this to happen and since the props object was truthy, it would try to clone the text node which caused an invariant error.

The first fix is to check the length the keys array for the props object instead of the truthiness of the props object itself. This allows text nodes to be rendered unless there are props to merge which would still cause an error. In this case we want to wrap the text node so any lingering props can be passed to the wrapping component.

Modified the sample to expose this.

[eightypop]

#52 tests for this case and the current build fails it.