NKF.nkfが内部で呼ばれるが、デコード途中で文字列が切れてしまうパターン

irb(main):001:0> source = "=?ISO-2022-JP?B?GyRCIXcbKEJJVBskQj83Q2VCLkpzGyhCIC0=?= =?ISO-2022-JP?B?IDIwMTAvOS8zGyRCISEbKEI=?= =?ISO-2022-JP?B?GyRCOkdNJUBoO3Y5YCRPIUhAZEJQJW0bKEI=?= =?ISO-2022-JP?B?GyRCITwlcyVBJDUkOyRrJDMkSCEqIUkbKEI=?="
=> "=?ISO-2022-JP?B?GyRCIXcbKEJJVBskQj83Q2VCLkpzGyhCIC0=?= =?ISO-2022-JP?B?IDIwMTAvOS8zGyRCISEbKEI=?= =?ISO-2022-JP?B?GyRCOkdNJUBoO3Y5YCRPIUhAZEJQJW0bKEI=?= =?ISO-2022-JP?B?GyRCITwlcyVBJDUkOyRrJDMkSCEqIUkbKEI=?="

irb(main):002:0> require 'kconv'
=> true

irb(main):003:0> source.toutf8()
error:
=> "＠IT新着速報 -"
except:
=> "＠IT新着速報 - 2010/9/3　最優先事項は“絶対ローンチさせること！”"

とりあえずデコードするためのメソッド

def decode_subject (subject)
  res = ""
  subject.split(' ').each do |word|
     res = res + NKF.nkf("-dJw", word)
  end
  return res
end