Move alert_box helper spec into spec/shared
Pass `alert_box` extra params to the alert <div>

Before this commit, the `alert_box` helper ignored unknown options.
For instance the code:

```rhtml
<%= alert_box message, priority: priority, class: 'important', id: 'alert' %>
```

would simply generate the following HTML

```html
<div class="alert alert-success" role="alert">
  ...
</div>
```

ignoring the `:class` and `:id` parameter. After this commit, the result is:

```html
<div class="important alert alert-success" id="alert" priority="notice" role="alert">
  ...
</div>
```