fix(lib/yukari.js): fix the bug: null is not equl to false(or 0, )

lib/yukari.js

@@ -426,6 +426,17 @@ Yukari.prototype.update = function(callback) {
             fieldType.$equal.bind(fieldType) :
             field.type.equal.bind(field.type));
 
+        // null not equal to false or 0 or ''
+        if((null === this[key] || null === this.$origData[key].data) && 
+            this[key] !== this.$origData[key].data && 
+            field.allowNull) {
+            data[field.name] = {
+                data    : this[key],
+                field   : field
+            };
+            continue;
+        }
+
         // only change changed data
         if(!equalFunc(this[key], this.$origData[key].data)) {
             data[field.name] = {