Conditional GET 有两种，一种是etag, 另一种是last-modified。
关于这两种介绍，请移步：http://www.dbanotes.net/web/http_11_etag_lastmodified.html
我们分别来看一下这两种实现。
1。用ETag

require 'open-uri'

 # 1st request -- save the ETag
 etag = nil
 open( "http://redhanded.hobix.com/index.xml" ) do |feed|
   etag = feed.meta['etag']
 end

 # 2nd request -- only retrieve the file if it has changed
 begin
   open( "http://redhanded.hobix.com/index.xml",
         "If-None-Match" => etag ) do |feed|
     puts "File has changed: #{ feed.read.length } bytes read"
   end
 rescue OpenURI::HTTPError
   puts "No file available or file has not changed."
 end

2。用last-modified

require 'open-uri'

 # 1st request -- save the modification time
 mtime = nil
 open( "http://redhanded.hobix.com/index.xml" ) do |feed|
   mtime = feed.last_modified
 end

 # 2nd request -- only retrieve the file if it has changed
 begin
   open( "http://redhanded.hobix.com/index.xml",
         "If-Modified-Since" => mtime.rfc2822 ) do |feed|
     puts "File has changed: #{ feed.read.length } bytes read"
   end
 rescue OpenURI::HTTPError
   puts "No file available or file has not changed."
 end