title: Solr stats facet left out some values
body: <p>I want to facet over products in Solr and want to retrieve statistics about the categories.
  My schema has got a field called categories, a multiValued field.
  the problem is, that the statistics left out a bunch of categories. Am I wrong with following query? Obviously, but how do I make it right? Pleeeease help, I'm despairing.</p>
  
  <pre><code>q=*:*&amp;rows=0&amp;stats=true&amp;stats.field=product_id&amp;rows=0&amp;stats.facet=categories
  </code></pre>