update validateIncludeElement to support nested include statements

test/include.test.js

+/* jshint camelcase: false */
+/* jshint expr: true */
+var chai      = require('chai')
+  , Sequelize = require('../index')
+  , expect    = chai.expect
+  , Support   = require(__dirname + '/support')
+  , DataTypes = require(__dirname + "/../lib/data-types")
+  , dialect   = Support.getTestDialect()
+  , config    = require(__dirname + "/config/config")
+  , sinon     = require('sinon')
+  , datetime  = require('chai-datetime')
+  , _         = require('lodash')
+  , moment    = require('moment')
+  , async     = require('async')
+
+chai.use(datetime)
+chai.Assertion.includeStack = true
+
+describe(Support.getTestDialectTeaser("Include"), function () {
+  it('should support a simple nested belongsTo -> belongsTo include', function (done) {
+    var Task = this.sequelize.define('Task', {})
+      , User = this.sequelize.define('User', {})
+      , Group = this.sequelize.define('Group', {})
+
+    Task.belongsTo(User)
+    User.belongsTo(Group)
+
+    this.sequelize.sync().then(function () {
+      async.auto({
+        task: function (callback) {
+          Task.create().done(callback)
+        },
+        user: function (callback) {
+          User.create().done(callback)
+        },
+        group: function (callback) {
+          Group.create().done(callback)
+        },
+        taskUser: ['task', 'user', function (callback, results) {
+          results.task.setUser(results.user).done(callback)
+        }],
+        userGroup: ['user', 'group', function (callback, results) {
+          results.user.setGroup(results.group).done(callback)
+        }]
+      }, function (err, results) {
+        expect(err).not.to.be.ok
+
+        Task.find({
+          id: results.task.id,
+          include: [
+            {model: User, include: [
+              {model: Group}
+            ]}
+          ]
+        }).done(function (err, task) {
+          expect(task.user).to.be.ok
+          expect(task.user.group).to.be.ok
+        })
+      })
+    })
+  })
+})
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