a = Mechanize.new
a.user_agent_alias = 'Windows IE 9'

この user_agent_alias= は、中では Mechanize::AGENT_ALIASES と言う定数を参照してUAを決めているっぽい

Mechanize::AGENT_ALIASES                                                                                                              
=> {"Mechanize"=>
  "Mechanize/2.7.3 Ruby/2.0.0p353 (http://github.com/sparklemotion/mechanize/)",
 "Linux Firefox"=>
  "Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.2.1) Gecko/20100122 firefox/3.6.1",
 "Linux Konqueror"=>"Mozilla/5.0 (compatible; Konqueror/3; Linux)",
 "Linux Mozilla"=>
  "Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.4) Gecko/20030624",
 "Mac Firefox"=>
  "Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.6; en-US; rv:1.9.2) Gecko/20100115 Firefox/3.6",
 "Mac Mozilla"=>
  "Mozilla/5.0 (Macintosh; U; PPC Mac OS X Mach-O; en-US; rv:1.4a) Gecko/20030401",
 "Mac Safari 4"=>
  "Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_6_2; de-at) AppleWebKit/531.21.8 (KHTML, like Gecko) Version/4.0.4 Safari/531.21.10",
 "Mac Safari"=>
  "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_2) AppleWebKit/534.51.22 (KHTML, like Gecko) Version/5.1.1 Safari/534.51.22",
 "Windows Chrome"=>
  "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/29.0.1547.32 Safari/537.36",
 "Windows IE 6"=>"Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1)",
 "Windows IE 7"=>
  "Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.1.4322; .NET CLR 2.0.50727)",
 "Windows IE 8"=>
  "Mozilla/5.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0; .NET CLR 1.1.4322; .NET CLR 2.0.50727)",
 "Windows IE 9"=>
  "Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0)",
 "Windows Mozilla"=>
  "Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:1.4b) Gecko/20030516 Mozilla Firebird/0.6",
 "iPhone"=>
  "Mozilla/5.0 (iPhone; U; CPU like Mac OS X; en) AppleWebKit/420+ (KHTML, like Gecko) Version/3.0 Mobile/1C28 Safari/419.3",
 "iPad"=>
  "Mozilla/5.0 (iPad; U; CPU OS 3_2 like Mac OS X; en-us) AppleWebKit/531.21.10 (KHTML, like Gecko) Version/4.0.4 Mobile/7B334b Safari/531.21.10",
 "Android"=>
  "Mozilla/5.0 (Linux; U; Android 3.0; en-us; Xoom Build/HRI39) AppleWebKit/534.13 (KHTML, like Gecko) Version/4.0 Safari/534.13",
 "Mac FireFox"=>
  "Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.6; en-US; rv:1.9.2) Gecko/20100115 Firefox/3.6",
 "Linux FireFox"=>
  "Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.2.1) Gecko/20100122 firefox/3.6.1"}

IE 10 とかが無いのでプルリクチャンスですね。！