utils.appendChild mutates child fragment

When calling `utils.appendChild` with a child fragment, the fragment is mutated, resulting in an empty array.

```js
const jml = ['div'];
const fragment = ['', 'hello world'];
utils.appendChild(jml, fragment);
// jml === ['div', 'hello world']
// fragment === []
```

Ideally, the fragment should not be mutated.
See discussion: https://github.com/benjycui/jsonml.js/pull/5#discussion_r146114631