instance method transform_keys!

# File activesupport/lib/active_support/hash_with_indifferent_access.rb, line 349
    def transform_keys!(hash = NOT_GIVEN, &block)
      return to_enum(:transform_keys!) if NOT_GIVEN.equal?(hash) && !block_given?

      if hash.nil?
        super
      elsif NOT_GIVEN.equal?(hash)
        keys.each { |key| self[yield(key)] = delete(key) }
      elsif block_given?
        keys.each { |key| self[hash[key] || yield(key)] = delete(key) }
      else
        keys.each { |key| self[hash[key] || key] = delete(key) }
      end

      self
    end