text: Title: Can't query by id?
  Body: 
  Thanks for a great library.
  
  After starting examples/flask_mongoengine :
  I expected I would be able to query by id?
  I'm unable to query by id, but can query by name
  
  ```
  {
    allEmployees(id:"RW1wbG95ZWU6NWE5ZjE4NzA2MGJmZmVhZDcyYmI3Zjgw") {
      pageInfo {
        hasNextPage
      }
      edges {
        node {
          id, name        
          department {
            id, name
          }
        }
      }
    }
  }
  
  
  {
    "errors": [
      {
        "message": "u'RW1wbG95ZWU6NWE5ZjE4NzA2MGJmZmVhZDcyYmI3Zjgw' is not a valid ObjectId, it must be a 12-byte input or a 24-character hex string",
        "locations": [
          {
            "column": 3,
            "line": 13
          }
        ]
      }
    ],
    "data": {
      "allEmployees": null
    }
  }
  
  ```
  
  
  ```
  {
    allEmployees( name: "Peter") {
      pageInfo {
        hasNextPage
      }
      edges {
        node {
          id, name        
          department {
            id, name
          }
        }
      }
    }
  }
  
  {
    "data": {
      "allEmployees": {
        "pageInfo": {
          "hasNextPage": false
        },
        "edges": [
          {
            "node": {
              "id": "RW1wbG95ZWU6NWE5ZjE4NzA2MGJmZmVhZDcyYmI3Zjgw",
              "name": "Peter",
              "department": {
                "id": "RGVwYXJ0bWVudDo1YTlmMTg3MDYwYmZmZWFkNzJiYjdmN2M=",
                "name": "Engineering"
              }
            }
          }
        ]
      }
    }
  }
  ```