Remove empty CommonPrefixes stanza

This is not needed and confuses s3-tests.

src/main/java/org/gaul/s3proxy/S3ProxyHandler.java

@@ -1024,7 +1024,7 @@ final class S3ProxyHandler extends AbstractHandler {
                 xml.writeEndElement();
             }
 
-            xml.writeEmptyElement("CommonPrefixes");
+            // TODO: CommonPrefixes
 
             xml.writeEndElement();