maxthon_history_xcs.rb - fix User-agent string

request.headers['User-agent'] is incorrect, it should be
request.headers['User-Agent'].

Downloaded following version from oldapps.com to confirm
the exploit code is wrong.

Supported Systems Windows 98, 2000 (Maxthon 2.5.15 Build
  1000), XP, Vista, 7, 8
MD5 Checksum F3791637C886A46940876211209F82F4
SHA1 Checksum 039BB218245E5DC1BAB0F57298C68AC487F86323

Release Date 20 October, 2011 (2 years ago )