docs: SimpleJsonEncoder appendToJSON(final String key, final Object value) {
          if (closed) {
              throw new IllegalStateException("Encoder already closed");
          }
          if (value != null) {
              appendKey(key);
              if (value instanceof Number) {
                  sb.append(value.toString());
              } else {
                  sb.append(QUOTE).append(escapeString(value.toString())).append(QUOTE);
              }
          }
          return this;
      }