CamelCaseControllerConfigAttribute.cs

public class CamelCaseControllerConfigAttribute : Attribute, IControllerConfiguration
{
    public void Initialize(HttpControllerSettings controllerSettings, HttpControllerDescriptor controllerDescriptor)
    {
        var formatter = controllerSettings.Formatters.OfType<JsonMediaTypeFormatter>().Single();
        controllerSettings.Formatters.Remove(formatter);

        formatter = new JsonMediaTypeFormatter
        {
            SerializerSettings = { ContractResolver = new CamelCasePropertyNamesContractResolver() }
        };

        controllerSettings.Formatters.Add(formatter);

    }
}

Controller.cs

[CamelCaseControllerConfig]
public dynamic Get()
{
    return new
    {
        Nome = "Jeferson",
        Sobrenome = "Bueno",
        Idade = 2000
    };
}