Change icon helper input params

Before:
```ruby
<%= icon :spinner, nil, class: "fa-spin" %>
#=> <i class="fa fa-spinner fa-spin"></i>

<%= icon :spinner, "Loading", class: "fa-spin" %>
#=> <i class="fa fa-spinner fa-spin">Loading</i>
```
After
```ruby
<%= icon :spinner, class: "fa-spin" %>
#=> <i class="fa fa-spinner fa-spin"></i>

<%= icon :spinner, "Loading", class: "fa-spin" %>
#=> <i class="fa fa-spinner fa-spin">Loading</i>
```