Fix Mechanize::Form::Field#inspect to read from accessor methods

This fixes `Mechanize::Form::Field#inspect` to read from accessor methods rather than directly from instance variables.  Without this fix, `#inspect` ends up displaying incorrect attribute values when the accessor methods are overridden by subclasses such as `SelectList`.

For instance in the following test case:

```html
<form name="form1" method="post" action="/form_post">
  <select name="select">
    <option value="1">Option 1</option>
    <option value="2" selected>Option 2</option>
    <option value="3">Option 3</option>
    <option value="4">Option 4</option>
    <option value="5">Option 5</option>
    <option value="6">Option 6</option>
  </select>
</form>
```

* Before fix: `[selectlist:0x817657d8 type:  name: select value: ]`
* After fix: `[selectlist:0x817657d8 type:  name: select value: 2]`