Add support for root as host node

[eddyerburgh]

Currently we only support the root being a Functional component, or a Class component.

There is code to handle root host nodes, but it's buggy and there are tests that aren't passing.

See this test for an example—https://github.com/bbc/enzyme-adapter-inferno/blob/master/test/specs/ReactWrapper.spec.js#L283

[eddyerburgh]

This wasn't an issue with host node support.

The issue was that enzyme was comparing a raw vnode with a converted RST node. This PR fixes this issue—enzymejs/enzyme#1423