def []=(name, value)
      if name.to_s == 'body'
        self.body = value
      elsif name.to_s =~ /content[-_]type/i
        header[name] = value
      elsif name.to_s == 'charset'
        self.charset = value
      else
        header[name] = value
      end
    end
def []=(name, value)
      if name.to_s == 'body'
        self.body = value
      elsif name.to_s =~ /content[-_]type/i
        header[name] = value
      elsif name.to_s == 'charset'
        self.charset = value
      else
        header[name] = value
      end
    end
Allows you to add an arbitrary header

 Example:

  mail['foo'] = '1234'
  mail['foo'].to_s #=> '1234'