fix issue kordamp#54 and add testcase

subprojects/json-lib-core/src/main/java/org/kordamp/json/AbstractJSON.java

@@ -242,7 +242,7 @@ private static Set getCycleSet() {
     }
 
     protected Object _processValue(Object value, JsonConfig jsonConfig) {
-        if (JSONNull.getInstance().equals(value)) {
+        if (JSONNull.getInstance().equals(value) && !(value instanceof String)) {
             return JSONNull.getInstance();
         } else if (Class.class.isAssignableFrom(value.getClass()) || value instanceof Class) {
             return ((Class) value).getName();

subprojects/json-lib-core/src/main/java/org/kordamp/json/util/JSONUtils.java

@@ -327,8 +327,7 @@ public static boolean isNull(Object obj) {
         if (obj instanceof JSONObject) {
             return ((JSONObject) obj).isNullObject();
         }
-        return JSONNull.getInstance()
-            .equals(obj);
+        return obj instanceof JSONNull;
     }
 
     /**

subprojects/json-lib-core/src/test/java/org/kordamp/json/bugs/TestNullString.java

@@ -0,0 +1,52 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Copyright 2006-2019 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.kordamp.json.bugs;
+
+import junit.framework.TestCase;
+import org.kordamp.json.Assertions;
+import org.kordamp.json.JSONArray;
+import org.kordamp.json.JSONNull;
+import org.kordamp.json.JSONObject;
+
+public class TestNullString extends TestCase {
+    public void test_object_from_null_string() {
+        JSONObject object = JSONObject.fromObject("{key:\"null\"}");
+        Assertions.assertEquals("{\"key\":\"null\"}", object.toString());
+        Assertions.assertTrue(object.get("key") instanceof String);
+
+        object = JSONObject.fromObject("{key:null}");
+        Assertions.assertEquals("{\"key\":null}", object.toString());
+        Assertions.assertTrue(object.get("key") instanceof JSONNull);
+    }
+
+    public void test_object_element_null_string() {
+        JSONObject object = new JSONObject();
+        object.element("key", "null");
+        Assertions.assertTrue(object.get("key") instanceof String);
+    }
+
+    public void test_array_from_null_string() {
+        JSONArray array = JSONArray.fromObject("[\"null\"]");
+        Assertions.assertEquals("[\"null\"]", array.toString());
+        Assertions.assertTrue(array.get(0) instanceof String);
+
+        array = JSONArray.fromObject("[null]");
+        Assertions.assertEquals("[null]", array.toString());
+        Assertions.assertTrue(array.get(0) instanceof JSONNull);
+    }
+}