title: Sunspot Solr search parameter "with" not working with strings
body: <p>I'm developing a Ruby on Rails application using Sunspot Solr as an indexer.
  The thing is i try to use the parameter of the search with :, using strings and it doesn't seem to work. If i pass an int it works fine. Anyone knows how to do it with strings (if it's possible)?</p>
  
  <p>An example of the search i want to do is:</p>
  
  <pre><code>@search = Sunspot.search(Record) do
       fulltext params[:query]
       with :checked, "Checked"
  end
  </code></pre>
  
  <p>:checked is an attribute of the table Record and it validates if a record is checked so it can't be edited.</p>
  
  <p>PD: I'm doing it this way because MySQL doesn't accept booleans. </p>