Exception thrown when updating a text node

[cjohansen]

Related t #971. I'm not sure what the intended behavior for text nodes is, but I had hoped this would work:

var el = document.createElement('div');
document.body.appendChild(el);

// Works as expected
var vdom = patch(el, snabbdom.vnode(undefined, {}, [], 'Text', null));

// Throws exception
var vdom2 = patch(vdom, snabbdom.vnode(undefined, {}, [], 'Text 2', null));

The second patch call throws an exception where it tries to do getAttribute on null. Is this even supposed to work?

Changing an h2 to a bare text works:

var vdom = patch(el, snabbdom.vnode("h2", {}, [], 'Text', null));
patch(vdom, snabbdom.vnode(undefined, {}, [], 'Text 2', null));

But the other way around does not:

var vdom = patch(vdom, snabbdom.vnode(undefined, {}, [], 'Text 2', null));
patch(el, snabbdom.vnode("h2", {}, [], 'Text', null));