Fixing $in, $nin, $all when using string

This patch correct a bug when requesting with string list conditions.

var Engine = require('tingodb')()
var db = new Engine.Db('./test', {});
var collection = db.collection("test");

collection.insert([{field:"A"},{field:"B"},{field:"C"}],function(){
	collection.find({field:{$in: ["A","C"]}}).count(function(err,cnt){
		console.log(cnt);
	});
});

before : undefined
after : 2