Web Api returns expanded key-value object instead of original JSON object

When I send {"name":"John Doe", "age":18, "country":"USA"} to my C# Web API with POST to api/test, I store it in my mongo test-collection and return the updated document:

[HttpPost]
[Route("{collection}")]
public IHttpActionResult Upsert(string collection, HttpRequestMessage request)
{
    var document = request.Content.ReadAsStringAsync().Result;
    var doc = BsonDocument.Parse(document);
    var result = new Db(collection).Upsert(doc).Result;
    return Ok(result);
}

.

public async Task<BsonDocument> Upsert(BsonDocument document)
{
    if (document.Contains("_id"))
    {
        await _collection.ReplaceOneAsync(w => w["_id"] == document["_id"], document);
    }
    else
    {
        await _collection.InsertOneAsync(document);
    }
    return document;
}

This works, but the result is now a key-value object:

[
  {
    "_name": "_id",
    "_value": "56e9364d942e1f287805e170"
  },
  {
    "_name": "name",
    "_value": "John Doe"
  },
  {
    "_name": "age",
    "_value": 18
  },
  {
    "_name": "country",
    "_value": "USA"
  }
]

What I would expect is:

{
    "_id": "56e9364d942e1f287805e170", 
    "name":"John Doe", 
    "age":18, 
    "country":"USA"
}

How can I achieve this?