mechanize：Mechanize#pluggable_parser

>> require 'mechanize'
=> true
>> agent = Mechanize.new
=> #<Mechanize:0xb74281d4
  ...
>> agent.pluggable_parser
=> #<Mechanize::PluggableParser:0xb742815c @default=Mechanize::File, @parsers={"application/xhtml+xml"=>Mechanize::Page, "text/html"=>Mechanize::Page, "application/vnd.wap.xhtml+xml"=>Mechanize::Page}>