2.0.0: validation fails validator argument is array and validate() is called twice in a row.

In v2.0.0:

Say we define validation on a field:
```javascript
{
  foo: {
    type: Sequelize.STRING,
    validations:{
      isIn: [['a', 'b']]
    }
  }
}
```

validating a model twice will fail the second time because isIn argument is modified by prepending `next` callback.