def to_s!(&block)
      blk = block || TO_RAW
      @headers.map { |header| header.to_s!(&blk) }.join("\n")
    end
def to_s!(&block)
      blk = block || TO_RAW
      @headers.map { |header| header.to_s!(&blk) }.join("\n")
    end
If a block is given, passes the block to Header#to_s! Joins
 the result with newlines.