class Mixlib::ShellOut

def umask=(new_umask)

will be converted to an integer by String#oct.
Set the umask that the subprocess will have. If given as a string, it 
def umask=(new_umask)
  @umask = (new_umask.respond_to?(:oct) ? new_umask.oct : new_umask.to_i) & 007777
end