title: rails + simpleform: selected does not set to be enum's value
body: <p>I have a enum definition for my User model.</p>
  
  <pre><code>class User &lt; ActiveRecord::Base
    enum program_of_study: [
      :program_of_study_unknown, :program_of_study_cs, :program_of_study_ceg,
      :program_of_study_is, :program_of_study_science,
      :program_of_study_engineering, :program_of_study_fass,
      :program_of_study_business, :program_of_study_others
    ]
  end
  </code></pre>
  
  <p>And in simple_form I have:</p>
  
  <pre><code>&lt;%= simple_form_for(locals[:user], wrapper: :horizontal_form, html: {class: 'form-horizontal'},
                      url: {action: (locals[:is_new] ? 'create' : 'update')}) do |f| %&gt;
      &lt;%= f.error_notification %&gt;
      &lt;%= f.input :program_of_study, collection: User.program_of_studies, include_blank: false, selected: locals[:user].program_of_study %&gt;
      &lt;%= f.button :submit, class: 'btn-success' %&gt;
  &lt;% end %&gt;
  </code></pre>
  
  <p>However, it seems that although the <code>program_of_study</code> of a user is <code>'program_of_study_science'</code>(by checking in rails console), when I render the form, the shown select element still has <code>'program_of_study_unknown'</code> as the displayed one. The correct one was not selected.</p>