title: Loggin Session issue with field prepolated already
body: <p>I have a little issue, I try to create a loggin session and in work  almost well the issue is in when I run the loggin view the fields have information already and I want that these field are in blank.</p>
  
  <pre><code>&lt;h1 class="cuadro_titulo container letramenu"&gt;Log in&lt;/h1&gt;
  
  &lt;section class = "fondo_form"&gt;
    &lt;div class="row"&gt;
      &lt;div class="col-md-6 col-md-offset-3 cuadro_info"&gt;
        &lt;%= form_for(:session, url: login_path) do |f| %&gt;
          &lt;div&gt;
             &lt;%= f.label :usuario, class: "Etiqueta_campo"  %&gt;
             &lt;p&gt;
               &lt;%= f.text_field :usuario, :value =&gt; nil, placeholder: "Escribe 
                  aqui usuario", class: "campo_peq" %&gt;
             &lt;/p&gt;
          &lt;/div&gt;
  
          &lt;%= f.label :password %&gt;
          &lt;%= f.password_field :password, :value =&gt; "", autocompleate: "off" 
           %&gt;
  
          &lt;%= f.submit "Log in", class: "btn btn-primary campo_peq centrar" %&gt;
        &lt;% end %&gt;
  
      &lt;/div&gt;
    &lt;/div&gt;
  &lt;/section&gt;
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/PcU9o.jpg" rel="nofollow noreferrer">enter image description here</a></p>
  
  <p>I will apriciate any idea,  thanks</p>