Buble's spread will not work on array-like (i. e. iterable) objects

[Spreading a NodeList into an array works in vanilla JS](https://jsfiddle.net/yeL4x7v4/). The following example will log an _array_ containing text, p, text, p, text.

```html

```js
console.log([...document.querySelector('div').childNodes])
```

However, because Buble uses `array.concat` for spread, [it will fail on _array-like_ objects](https://buble.surge.sh/#const%20foo%20%3D%20%5B...someElement.childNodes%5D) because concat will only "spread" arrays. Anything else, it just appends to the target array as is.

Input:

```js
const foo = [...someElement.childNodes] // Expected [node, node, node, ...]
```
Output:

```js
var foo = [].concat(someElement.childNodes) // Actual [NodeList{}]
```

Start with the linked vanilla-JS example and Buble reproduction, then trace the spread transformation that produces [].concat(someElement.childNodes). Done means array-like iterable values are expanded into individual elements, with a regression test covering the NodeList case and the expected array output.

Written by the indexing model from the issue text.