Use IdentityHashSet for cycle detection

Fixes https://github.com/stleary/JSON-java/issues/650

src/test/java/org/json/junit/data/RecursiveBeanEquals.java

@@ -0,0 +1,33 @@
+package org.json.junit.data;
+
+/** test class for verifying if recursively defined bean can be correctly identified */
+public class RecursiveBeanEquals {
+  private final String name;
+  private Object reference;
+
+  public RecursiveBeanEquals(String name) {
+    this.name = name;
+  }
+
+  public String getName() {
+    return name;
+  }
+
+  public Object getRef() {
+    return reference;
+  }
+
+  public void setRef(Object refObj) {
+    reference = refObj;
+  }
+
+  @Override
+  public boolean equals(Object other) {
+    return other instanceof RecursiveBeanEquals && name.equals(((RecursiveBeanEquals) other).name);
+  }
+
+  @Override
+  public int hashCode() {
+    return name.hashCode();
+  }
+}