PATCH request with nullable attributes does not work

Hi,
Thanks for this great library!

When you are trying to PATCH a model with nullable attributes, it does not work.

Model example:

public class TodoItem : Identifiable
{
    Attr("description")]
    public string Description { get; set; }

    [Attr("created-date")]
    public DateTime CreatedDate { get; set; }

    [Attr("achieved-date")]
    public DateTime? AchievedDate { get; set; }
}

1st PATCH Request example: Field is not sent
PATCH http://localhost:5000/api/todoitem/1

{
  "description": "New description"
}

2nd PATCH Request example: Field is sent
PATCH http://localhost:5000/api/todoitem/1

{
  "description": "New description",
  "achieved-date": "2017-04-25T21:41:29.033Z"
}

Actual behavior: Exception is returned from the API.
Expected behavior:

• Field is ignored if not sent (1st example)
• Field is updated with new value if sent (2nd example)

Gerfaut