Small Bugfix: getter setter property initialization

this PR fixes an edge case for intialization of data-property getters & setters. It removes a bogus (unnecessary) check for whether the underlying `dataValue` is set. the following snippet show cases an example where getter/setter initialization breaks (for the "slug" property):

```js
var Product = this.sequelize.define('ProductWithSettersAndGetters', {
  name: {
    type: Sequelize.STRING,
    set : function(v) {
      this.setDataValue('slug', _.slugify(v));
      this.setDataValue('name', v);
    }
  },
  slug : {
    type: Sequelize.STRING,
 }
});
```

sorry no tests - I hope to develop related tests as a result of the discussions in issue #675