Nav uses "incorrect" HTML for caret

Currently `yii\bootstrap\Nav` uses the following default HTML for Bootstrap caret:

``` HTML
<b class="caret"><b>
```

This is not in line with Bootstrap spec itself (see http://getbootstrap.com/css/#helper-classes-carets) and also inconsistent with (correct) HTML used in `yii\bootstrap\ButtonDropdown`:

``` PHP
} else { 
$label .= ' <span class="caret"></span>'; 
$options = $this->options; 
```
