reset fields on update attributes

When you call ```updateAttributes``` after ```create```, fields are kept from the create method.
btw I think that this pull also solve #1320 but didn't test it


Here is the bug code example that this pull solve
```
var Sequelize = require('sequelize');

var sequelize = new Sequelize(dbname, username, password, settings);

var Project = sequelize.define('Project', {
  title: Sequelize.STRING,
  description: Sequelize.TEXT,
  foo: Sequelize.STRING
});

sequelize.sync({force: true}).then(function() {
	Project.create({title: 'test', description: 'desc'}, {fields: ['title', 'description']}).then(function(project) {
		return project.updateAttributes({foo: 'bar'})
	}).then(function(project){
		// project.foo is undefined because this.options.attributes are ['title', 'description'] 
	});
});
```