title: duplicated function fails for ff date vectors
body: <p>Hi I am trying to remove duplicates from a ff vector that contains dates using the duplicated function of the ffbase package and the following code:</p>
  
  <pre><code>v1 &lt;- c("24-Mar-94", "24-Mar-94", "27-Mar-94", "28-Jun-1986", "29-Jul-1988", "28-Jun-1986", "15-Jan-1999", "13-Jan-1999")
  v1.d &lt;- as.Date(v1, format="%d-%b-%y")
  v1.ff &lt;- as.ff(v1.d)
  v2 &lt;- v1.ff[!duplicated(v1.ff)]
  </code></pre>
  
  <p>However I get the following error:</p>
  
  <pre><code>Error in UseMethod("as.hi") : 
    no applicable method for 'as.hi' applied to an object of class "Date"
  </code></pre>
  
  <p>Is there any way around this problem without having to coerce the v2 vector to a ram object first?</p>