title: Formtastic belongs_to select tag generates a blank selection option
body: <p>I'm using Formtastic as a semantic_form_for books and lists. Each list <code>has_many</code> books and each book <code>belongs_to</code> a list. When the user creates a new book, they can choose what list to add the book to. Here is my form code:</p>
  
  <pre><code>&lt;%= semantic_form_for(@book) do |f| %&gt;
      &lt;p&gt;Select a list to add this book to: &lt;/p&gt;
      &lt;%= f.input :list, :as =&gt; :select %&gt;
      ...
      &lt;%= f.submit "Add book", class: "btn btn-default" %&gt;
  &lt;% end %&gt;
  </code></pre>
  
  <p>The only problem is that the select tag that is generated has a blank option for select at the top. So the options for select look like this:</p>
  
  <ul>
  <li>[blank]</li>
  <li>List 1</li>
  <li>List 2</li>
  <li><p>List 3</p>
  
  <p>etc.</p></li>
  </ul>
  
  <p>I don't want there to be an extra space. How can I fix that?</p>
  
  <p>Here is a picture:</p>
  
  <p><a href="https://i.stack.imgur.com/b0xlp.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/b0xlp.png" alt="enter image description here"></a></p>