Fix #141.

@domenic wrote:
> It would be nice to remove `document.outerHTML`, `document.innerHTML`, and any serialization-related `toString`s from jsdom before the 1.0 release, as part of the breaking changes batch.

It looks like `document.outerHTML` is already completely gone, and I don't see any `toString` methods being used for serialization.

That leaves `document.innerHTML`, and this PR takes care of that.