Handle trailing slash during HEAD bucket

Found with Ceph s3-tests.  References #5.

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

@@ -156,7 +156,7 @@ final class S3ProxyHandler extends AbstractHandler {
             }
         case "HEAD":
             if (path.length <= 2 || path[2].isEmpty()) {
-                handleContainerExists(response, uri.substring(1));
+                handleContainerExists(response, path[1]);
                 baseRequest.setHandled(true);
                 return;
             } else {