Fix overloading of the `in` operator for this.rows

Currently there is a draft implementation of the overloading of the `in` operator but doesn't seem to work properly. I got it to work with a string dataframe but not numeric. You should be able to:

```js
const hasRow = [1,2,3] in df;
```

You need to use the proxy that is already returned from the `DataFrame.rows` getter.