Fix KeyError when signing http_redirect_message

Without this change I get the following traceback:

>>> pack.http_redirect_message(req.to_string(), destination, sigalg=pack.RSA_SHA1, key=rsa)
Traceback (most recent call last):
  File "<redacted>", line 1, in <module>
  File "<redacted>\pysaml2-1.0.3-py2.7.egg\saml2\pack.py", line 138, in http_redirect_message
    string = "&".join([urllib.urlencode({k: args[k]}) for k in _order])
KeyError: 'RelayState'