Restore old API for merge.

Author: uhafner

src/main/java/edu/hm/hafner/coverage/Node.java

@@ -694,7 +694,7 @@ private String createTargetClassName(final ClassNode testClassNode) {
      *
      * @return a new tree with the merged {@link Node nodes}
      */
-    public static Node merge(final List<Node> nodes) {
+    public static Node merge(final List<? extends Node> nodes) {
         if (nodes.isEmpty()) {
             throw new IllegalArgumentException("Cannot merge an empty list of nodes");
         }
@@ -707,6 +707,7 @@ public static Node merge(final List<Node> nodes) {
 
         if (grouped.size() == 1) {
             return nodes.stream()
+                    .map(Node.class::cast)
                     .reduce(Node::merge)
                     .orElseThrow(() -> new NoSuchElementException("No node found"));
         }