only add id (null) if not passed as value

lib/sequelize/model.js

@@ -7,10 +7,12 @@ var Model = module.exports = function(values) {
   
   this.definition = null // will be set on Model.build or Model.create
   this.attributes = []
-  this.addAttribute('id', null) // a newly created model has no id
-  
+
   // add all passed values to the model and store the attribute names in this.attributes
   Utils._.map(values, function(value, key) { self.addAttribute(key, value) })
+  
+  if(!this.hasOwnProperty('id'))
+    this.addAttribute('id', null) // a newly created model has no id
 }
 Utils.addEventEmitter(Model)
 Utils._.map(Mixin.classMethods, function(fct, name) { Model[name] = fct })