title: Cast Map<String, ? extends Object> to Map<String, Object>
body: <p>In Java 8, I am trying to cast <code>Map&lt;String, ? extends Object&gt;</code> to <code>Map&lt;String, Object&gt;</code>. I though that it would be safe given the constraint I put on the input Type (all ? must implement Object), but I get an unchecked cast warning.</p>
  
  <p>Any idea where I reason wrong ? Any clean solution ? Thanks for your help !</p>