Create an "Authorization" header value from an authdict (created by generate_response()).

def build_authorization_arg(authdict):
  """
  Create an "Authorization" header value from an authdict (created by generate_response()).
  """
  vallist = []
  for k in authdict.keys():
    vallist += ['%s=%s' % (k,authdict[k])]
  return 'Digest '+', '.join(vallist)