title: Sonata admin: setting a filter to false by default?
body: <p>I am currently using Sonata Admin to generate a datagrid with an entity having a <code>read</code> boolean field. I would like to filter on this property, setting it by default to <code>false</code>.</p>
  
  <p>So, I added the following to my Admin class:</p>
  
  <pre><code>protected $datagridValues = array(
      'read' =&gt; array('value' =&gt; false),
  );
  </code></pre>
  
  <p>Yet, it does not seem to work. The generated select list is the following:</p>
  
  <pre><code>&lt;select id="filter_read_value" name="filter[read][value]" class="span8"&gt;
      &lt;option value=""&gt;&lt;/option&gt;
      &lt;option value="1"&gt;oui&lt;/option&gt;
      &lt;option value="2"&gt;non&lt;/option&gt;
  &lt;/select&gt;
  </code></pre>
  
  <p>I suppose this is normal, as <code>value</code> for false would be 0, which is the empty option.</p>
  
  <p>So, I used some constants such as:</p>
  
  <pre><code>const STATUS_READ = 1;
  const STATUS_UNREAD = 2;
  </code></pre>
  
  <p>It works, but I am wondering if there is any proper solution to avoid these two unnecessary constants?</p>