code: public JsonUnflattener withSeparator(char separator) {
      isTrue(!Character.toString(separator).matches("[\"\\s]"),
          "Separator contains illegal chracter(%s)",
          Character.toString(separator));
      isTrue(!leftBracket.equals(separator) && !rightBracket.equals(separator),
          "Separator(%s) is already used in brackets",
          Character.toString(separator));
  
      this.separator = separator;
      return this;
    }
docs: A fluent setter to setup the separator within a key in the flattened JSON.
  The default separator is a dot(.).
  
  @param separator
  any character
  @return this {@link JsonUnflattener}