title: Custom columns name when exporting Sonata Admin
body: <p>I'm trying to export some columns and few of them have custom values like (0=closed, 1=pending, 2=under revision, 3=approved). It is possible to export their names: Closed / Pending etc instead of 0,1,2,3... ? How ?</p>
  
  <p>I tried using <code>getExportFields()</code> from Sonata, but all I did was to change the label.</p>
  
  <pre><code>Id     Name      Status                
   1     Item1       0
   2     Item2       3
  </code></pre>
  
  <p>to </p>
  
  <pre><code>Id     Name      Status                
  1      Item1     Closed
  2      Item2    Approved
  </code></pre>