Do not overwrite selected and disabled attributes

In an application I'm working on, I have the following select:

```
<select id="task_assigned_id" name="task[assigned_id]">
  <option type="Coach" value="1">Joseph O'Connor</option>
  <option type="Coachee" value="1">Joseph O'Connor</option>
</select>
```

To fill it, I'm generating an array of options with its attributes:

```
[
  ['Joseph O'Connor', 1, { type: 'Coach', selected: 'selected' }],
  ['Joseph O'Connor', 1, { type: 'Coachee' }]
]
```

One of these attributes is ```selected```, but the ```options_for_select``` method is ignoring it.
On the other hand I can't use the second argument of the function, cause it is always selecting the last option in the list.

The ```disabled``` attribute has the same problem.

This PR solves this issue, preventing this two attributes to be overwritten when they are explicitly specified.