Fix asBuffer Array test

This test doesn't appear correct to me.

Let's say I put an array of integers as a value:

```js
var db = require('level')('mydb')
db.put('foo', [1, 2, 3, 4])
```

If you call

```js
db.get('foo', {valueEncoding: 'binary'}, console.log)
```

Which of these two buffers would you expect to get back?

```
new Buffer([1, 2, 3, 4])
new Buffer("1,2,3,4")
```

I would tend to expect the first buffer, but instead I get back the second one.