fix crash caused by setting nil to a primitive attribute.

[khaledannajar]

When a wrong value "null" is set to a primitive attribute in the JSON and the DCAttributeSetter try to assign it to the attribute a crash is thrown

Here in this sample JSON book_id in int type and is set to null here I get the crash saying something like that " setNilValueForKey]: could not set nil as the value for the key _book_id.' ":

I fixed it by simply setting @0 to this special case.

{
"name": "andro",
"avatar": "http://url/userimage.jpg",
"summary": "pla pla pla ",
"book_id": null,
"datetime": 1455714077
}