Fix belongsTo where params

Usually when adding to a `foobar.getBla()` a where param
like `foobar.getBla({where: {x: "y"}})` it should extend
the existing where param (`"id": id`) with the additional
where param.
This didn't work for belongsTo associations until now.

lib/associations/belongs-to.js

@@ -42,8 +42,8 @@ module.exports = (function() {
       var id = this[self.identifier]
 
       if (!Utils._.isUndefined(params)) {
-        if (!Utils._.isUndefined(params.attributes)) {
-          params = Utils._.extend({where: {id:id}}, params)
+        if (!Utils._.isUndefined(params.where)) {
+          params.where = Utils._.extend({id:id}, params.where)
         }
       } else {
         params = id

spec-jasmine/associations/belongs-to.spec.js

@@ -8,7 +8,10 @@ describe('BelongsTo', function() {
     , Task = null
 
   var setup = function() {
-    User = sequelize.define('User', { username: Sequelize.STRING })
+    User = sequelize.define('User', { username: Sequelize.STRING, enabled: {
+      type: Sequelize.BOOLEAN,
+      defaultValue: true
+    }})
     Task = sequelize.define('Task', { title: Sequelize.STRING })
   }
 
@@ -88,6 +91,29 @@ describe('BelongsTo', function() {
     })
   })
 
+  it('extends the id where param with the supplied where params', function() {
+    Task.belongsTo(User, {as: 'User'})
+
+    Helpers.async(function(done) {
+      User.sync({force: true}).success(function() {
+        Task.sync({force: true}).success(done)
+      })
+    })
+
+    Helpers.async(function(done) {
+      User.create({username: 'asd', enabled: false}).success(function(u) {
+        Task.create({title: 'a task'}).success(function(t) {
+          t.setUser(u).success(function() {
+            t.getUser({where: {enabled: true}}).success(function(user) {
+              expect(user).toEqual(null)
+              done()
+            })
+          })
+        })
+      })
+    })
+  })
+
   it("handles self associations", function() {
     Helpers.async(function(done) {
       var Person = sequelize.define('Person', { name: Sequelize.STRING })