Issue #6558 - Allow configuring return type in JSON array parsing.

Updated JSON implementation to keep backward compatibility
by calling newArray(), now deprecated.

Signed-off-by: Simone Bordet <simone.bordet@gmail.com>

jetty-util-ajax/src/main/java/org/eclipse/jetty/util/ajax/JSON.java

@@ -33,6 +33,7 @@
 import java.util.Objects;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.function.Function;
+import java.util.stream.IntStream;
 
 import org.eclipse.jetty.util.IO;
 import org.eclipse.jetty.util.Loader;
@@ -97,7 +98,7 @@ public class JSON
 
     private final Map<String, Convertor> _convertors = new ConcurrentHashMap<>();
     private int _stringBufferSize = 1024;
-    private Function<List<?>, Object> _arrayConverter = List::toArray;
+    private Function<List<?>, Object> _arrayConverter = this::defaultArrayConverter;
 
     public JSON()
     {
@@ -1070,6 +1071,14 @@ protected Object parseObject(Source source)
         return map;
     }
 
+    private Object defaultArrayConverter(List<?> list)
+    {
+        // Call newArray() to keep backward compatibility.
+        Object[] objects = newArray(list.size());
+        IntStream.range(0, list.size()).forEach(i -> objects[i] = list.get(i));
+        return objects;
+    }
+
     protected Object parseArray(Source source)
     {
         if (source.next() != '[')