Fixed memory leak

Fixes #27

Author: slaskawi

src/main/java/mjson/Json.java

@@ -1125,7 +1125,7 @@ public static Schema schema(Json S, URI uri)
 
    public static class DefaultFactory implements Factory
    {
-      public Json nil() { return Json.topnull; }
+      public Json nil() { return new NullJson(); }
       public Json bool(boolean x) { return new BooleanJson(x ? Boolean.TRUE : Boolean.FALSE, null); }
       public Json string(String x) { return new StringJson(x, null); }
       public Json number(Number x) { return new NumberJson(x, null); }
@@ -1134,7 +1134,7 @@ public static class DefaultFactory implements Factory
       public Json make(Object anything)
       {
          if (anything == null)
-            return topnull;
+            return nil();
          else if (anything instanceof Json)
             return (Json)anything;
          else if (anything instanceof String)
@@ -1913,8 +1913,6 @@ public Json next() {
 
    }
 
-   static NullJson topnull = new NullJson();
-
    /**
     * <p>
     * Set the parent (i.e. enclosing element) of Json element.