Can't db.insert()

https://github.com/db-migrate/pg/blob/master/index.js#L564-L566

```javascript
arguments[index] = arguments[index].map(function (value) {
return typeof value === 'string' ? value : JSON.stringify(value);
});
```

What is this function doing? `db-migrate`#insert doesn't do anything with de-stringifying json

https://github.com/db-migrate/db-migrate-base/blob/7af3f80bad7fa5c64509a4435cce68891403c0d1/index.js#L339

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