ElementReference only added to rendered markup during first render

[egil]

After further investigation, it looks as if this issue is not solvable as the RenderTreeDiffBuilder doesn't include the element reference id during later renders.

The logic in Blazor assumes that the DOM tree is not replaced at each render, but simply updated.

This will probably be easily solvable with #48.

[egil]

This seems to have been solved.