Add tests for submitting meta

There is no test coverage for sending meta in request bodies.

For example:
```
POST /api/people HTTP/1.1

{
  "data": {
    "type": "people",
    "attributes": {
      "firstName": "John",
      "lastName": "Doe"
    }
  },
  "meta": {
    "root": {
      "child": {
        "key-A": "value-A",
        "key-B": "value-B"
      }
    }
  }
}
```