Include KeyCount in List V2 responses

Found via s3-tests.

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

@@ -1410,6 +1410,9 @@ public class S3ProxyHandler {
                         encodingType, prefix));
             }
 
+            if (isListV2) {
+                writeSimpleElement(xml, "KeyCount", String.valueOf(set.size()));
+            }
             writeSimpleElement(xml, "MaxKeys", String.valueOf(maxKeys));
 
             if (!isListV2) {