title: how to get sales rank from Amazon ItemSearch/ItemLookup
body: <p>I am trying to grab sales ranks from the Amazon product advertising API. I'm using the <a href="http://code.papercavalier.com/amazon_product/" rel="nofollow">amazon_product</a> gem.</p>
  
  <pre><code>request = AmazonProduct["us"]
  
  request.configure do |c|
    c.key    = AMAZON_KEY
    c.secret = AMAZON_SECRET
    c.tag    = TAG
  end
  
  #itemlookup
  response = request.find('B003XU6H8I', :response_group =&gt; 'SalesRank')
  response.find('Item').first
  =&gt; {"ASIN"=&gt;"B003XU6H8I"}
  </code></pre>
  
  <p>Why does the API return no sales rank for this item? Also, I've tried getting the SalesRank response group with an item search but they don't get returned. Is this a problem with the API?</p>