def set_content_type(type, params = {})
    @header['content-type'] = [type + params.map{|k,v|"; #{k}=#{v}"}.join('')]
  end
Sets the content type in an HTTP header.
The +type+ should be a full HTTP content type, e.g. "text/html".
The +params+ are an optional Hash of parameters to add after the
content type, e.g. {'charset' => 'iso-8859-1'}