PDO fetchAll和json_encode忽略查询的数据类型

I have executed the following code. The wierd thing is that fields like id_expediente and id_juzgado are defined as integers on my mysql database and for any reason json_encode($respuesta) is returning those fields as Strings.

I want to keep its data types(as were defined in mysql)

$sql = $this->con->prepare($consulta);
$ok = $sql->execute(array());
$query = $sql->fetchAll(PDO::FETCH_ASSOC);      
$respuesta = array  (
        "codigo"=>1001,
        "mensaje"=>$mensajes[1001],
        "contenido"=>$query
);
return json_encode($respuesta);

What json_encode($respuesta) shows:

    {
  "codigo": 1001,
  "mensaje": "",
  "contenido": [
    {

      "id_expediente": "11",
      "id_juzgado": "3",
      "nombre": "PEDRO",
      "apellido1": "SANTIAGO",
      "apellido2": "FERNANDEZ",
      "fecha_defuncion": "2014-11-11 12:45:34",
      "id_tipo_zona": "4",
      "fallecio_en": "DOMICILIO - EL BURGOS",
      "id_tipo_servicio": "1",
      "enterramiento": "EL BURGO",
      "empresa_id": "0",
      "compania": "ALMUDENA",
      "responsable": "SENSI MADRID",
      "responsable_tlf": "600102000",
      "creado_en": "2015-09-08 12:06:54",
      "actualizado_en": "2015-09-08 12:06:54",
      "id": "15",
      "id_empleado": "1",
      "estados_expediente": "1"
    },
    {
      "id_expediente": "345",
      "id_juzgado": "1",
      "nombre": "Juan",
      "apellido1": "Bartolome",
      "apellido2": null,
      "fecha_defuncion": "2015-09-08 12:06:54",
      "id_tipo_zona": "1",
      "fallecio_en": null,
      "id_tipo_servicio": "1",
      "enterramiento": "Malaga",
      "empresa_id": "2",
      "compania": null,
      "responsable": null,
      "responsable_tlf": "666777888",
      "creado_en": "2015-09-30 21:13:07",
      "actualizado_en": "2015-09-30 21:13:07",
      "id": "31",
      "id_empleado": null,
      "estados_expediente": "1"
    },
    {
      "id_expediente": "350",
      "id_juzgado": "1",
      "nombre": "Juan",
      "apellido1": "Bartolome",
      "apellido2": null,
      "fecha_defuncion": "2015-09-08 12:06:54",
      "id_tipo_zona": "1",
      "fallecio_en": null,
      "id_tipo_servicio": "1",
      "enterramiento": "Malaga",
      "empresa_id": "2",
      "compania": null,
      "responsable": null,
      "responsable_tlf": "666777888",
      "creado_en": "2015-09-30 21:13:15",
      "actualizado_en": "2015-09-30 21:13:15",
      "id": "32",
      "id_empleado": null,
      "estados_expediente": "1"
    },
    {
      "id_expediente": "355",
      "id_juzgado": "1",
      "nombre": "Jorge",
      "apellido1": "Bartolome",
      "apellido2": null,
      "fecha_defuncion": "2015-09-08 12:06:54",
      "id_tipo_zona": "1",
      "fallecio_en": null,
      "id_tipo_servicio": "1",
      "enterramiento": "Malaga",
      "empresa_id": "2",
      "compania": null,
      "responsable": null,
      "responsable_tlf": "666777888",
      "creado_en": "2015-09-30 21:13:31",
      "actualizado_en": "2015-09-30 21:13:31",
      "id": "33",
      "id_empleado": null,
      "estados_expediente": "1"
    },
    {
      "id_expediente": "400",
      "id_juzgado": "1",
      "nombre": "Javier",
      "apellido1": "Bartolome",
      "apellido2": "Bengochea",
      "fecha_defuncion": "2015-09-08 12:06:54",
      "id_tipo_zona": "1",
      "fallecio_en": null,
      "id_tipo_servicio": "1",
      "enterramiento": "Malaga",
      "empresa_id": "2",
      "compania": null,
      "responsable": null,
      "responsable_tlf": "666777888",
      "creado_en": "2015-09-30 21:15:39",
      "actualizado_en": "2015-09-30 21:15:39",
      "id": "34",
      "id_empleado": null,
      "estados_expediente": "1"
    }
    ]
}