modify_subject()
  Modify email Subject.
  
  This scanner can be used to modify email Subject.
  
  Usage: modify_subject('prefix',scanners)
     or: modify_subject(...).onlyif('virname')
  
  Where: 'prefix' is an prefix added into Subject.
         'virname' is a string, which defines any virus name.
           By default all virus names at least 1 character long: '.'
         For more information see modify_header() interscanner.
  
  For example original Subject line is:
    Subject: Testing mail
  and i use:
    modify_subject('[SPAM]', spamassassind(['localhost',783]))
  When spam will be found, header will be modifyed to:
    Subject: [SPAM] Testing mail