Serialization behavior for classes with DataContract attribute

[lazytom]

Most serializers will handle classes that are tagged with a DataContract attribute in such a way that members that are not tagged with the DataMember attribute are ignored. (see https://msdn.microsoft.com/en-us/library/ms733127(v=vs.110).aspx)

Jil currently does not behave this way. Is this considered a feature? If not, I'd be happy to try and contribute a PR to fix this.

Here's a test to illustrate what I am talking about:

    [DataContract]
    class _WithDataContract
    {
        [DataMember]
        public string A { get; set; }
        public string B { get; set; }
    }

    [TestMethod]
    public void DataContract()
    {
        {
            var json = JSON.Serialize(new _WithDataContract { A = "A", B = "B" });
            Assert.AreEqual("{\"A\":\"A\"}", json);
        }
    }