title: Issues with dropdown select_tag placeholder with prompt
body: <p>I'm having an issue with setting a placeholder within dropdown selection form using the rails select_tag helper. Using the prompt option a placeholder exists, but this method produces a dead duplicate value in the dropdown that you can't click:</p>
  
  <p><a href="https://i.stack.imgur.com/kJEov.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/kJEov.jpg" alt="duplicate_dropdown"></a></p>
  
  <p>Here is my code:</p>
  
  <pre><code>@posts = Post.all
  @categories = Post.uniq.pluck(:category)
  @prompt = "Select Category"
  
  if params[:category]
     @posts = Post.where(category: params[:category])
     @prompt = params[:category]
  end
  
  &lt;%= form_tag(h_path, :method =&gt; "get") do %&gt;
     &lt;%= select_tag 'category', options_for_select(@categories), {onchange: "this.form.submit();", prompt: @prompt} %&gt;
  &lt;% end %&gt;
  </code></pre>
  
  <p>Any ideas or suggestions regarding making a placeholder value work nicely in this context would be greatly appreciated. By nicely I mean something like the category dropdown select example on this page:
  <a href="http://www.joeabercrombie.com/category/audiobooks/" rel="nofollow noreferrer">http://www.joeabercrombie.com/category/audiobooks/</a></p>