title: remove a blank option of select field that was generated by SimpleForm
body: <p>I have this piece of code:</p>
  <pre><code>= f.input :category, :as =&gt; :select, :label =&gt; false, :collection =&gt; Choices[&quot;Categories&quot;]
  </code></pre>
  <p>Choices[&quot;Categories&quot;] is just a hash of key=&gt;value pairs.</p>
  <p>SimpleForm generates a select field with all needed options, but it also makes the first option blank.<br/> This blank option is present in all select fields that were generated by SimpleForm.</p>
  <p>But I don't want to have a blank option. Is there a way to get rid of it?</p>
  <p>Something like <code>:allow_blank_option =&gt; false</code>?</p>
  <p>I tried to make a presence validation of this attribute hoping that SimpleForm will detect it, but it didn't help.</p>