fix UTF16 BOM removal problem on ruby 1.9.2

hi ping,

here's a fix for the problem with the BOM removal regexp on ruby 1.9.2

it's tested on both

ruby-1.9.2-p180 [ x86_64 ]
ruby-1.8.7-p334 [ x86_64 ]

against an Exchange 2010 SP1 server with NTLM authentication

it seems there are further problems with the ntlm_http.rb on ruby 1.9.2, but i authenticated fine using latest httpclient and 'net/ntlm' portion of pyu-ntlm-http gem, so i will raise another bug for the other problems