title: Nested icons and Rails' link_to or button_to are not working with SLIM
body: <p>Why doesn't this work?</p>
  
  <pre><code> # Note the `do` block
   = button_to 'Accept', firefighters_approve_path(entry), class: 'button is-success' do
      i.fa.fa-thumbs-up
  </code></pre>
  
  <blockquote>
    <p>undefined method `stringify_keys' for String:0x007fec0bb7bcc8></p>
  </blockquote>
  
  <p>Why doesn't this work?</p>
  
  <pre><code># Note no `do` block
  = button_to 'Accept', firefighters_approve_path(entry), class: 'button is-success'
    i.fa.fa-thumbs-up
  </code></pre>
  
  <blockquote>
    <p>undefined method `stringify_keys' for String:0x007fec0bc86b40</p>
  </blockquote>
  
  <p>I want to use slim and fontawesome. What am I missing here?</p>