Bug: Hasura stringifies Custom Action outputs (at least for arrays).

We have a custom action with the following Output:
```graphql
type Output {
fields : [[String!]]
}
```

Our NodeJS server, when the custom action is called, returns this kind of results:
```js
{
"fields": [
["Key", "XXX"],
["Content-Type", "YYY"],
...
]
}
```

Yet, the browser receives this response:
```js
{
"my_action": {
"fields": "[[\"Key\", \"XXX\"], [\"Content-Type\", \"YYY\"]]",
"__typename": "Output"
}
}
```

This wasn't the case with Hasura 1.x.