title: Simple form: how to set values of a collection
body: <p>I have the following code below: </p>
  
  <pre><code>= simple_form_for :credits, url: "/accounts/#{@account.id}/topup" do |f|
    = f.input :amount, collection: [100,500,1000,5000,10000],as: :radio_buttons
    = f.button :submit
  </code></pre>
  
  <p>It works to set all values and labels in the collection. What I want is something like, </p>
  
  <pre><code>label: 100, value: 500
  </code></pre>
  
  <p>How would that happen?</p>