# File lib/rmail/header.rb, line 331
    def add(name, value, index = nil, params = nil)
      value = value.to_str
      if params
        value = value.dup
        sep = "; "
        params.each do |n, v|
          value << sep
          value << n.to_s
          value << '='
          v = v.to_s
          if v =~ /^\w+$/
            value << v
          else
            value << '"'
            value << v
            value << '"'
          end
        end
      end
      field = Field.new(name, value)
      index ||= @fields.length
      @fields[index, 0] = field
      self
    end