docs: public static Map mapKeys(Mapper mapper, Map map, boolean allowNull) {
          HashMap h = new HashMap();
          for (Object e : map.entrySet()) {
              Map.Entry entry = (Map.Entry) e;
              Object o = mapper.map(entry.getKey());
              if (allowNull || o != null) {
                  h.put(o, entry.getValue());
              }
          }
          return h;
      }