Handle empty RelayState

[voyc-jean]

Hi,

When a login is initiated from an IdP (only tested on G Suite), the Response will include an empty value for the RelayState parameter if a redirect is not specified on the IdP (in the case of G Suite, this is called the Start URL).

Should this library not perhaps check to see if the RelayState value is empty and, in the case that it is, redirect to the default SAML_LOGIN_REDIRECT path?

Please see this line.

OneLogin_Saml2_Error: Redirect to invalid URL: 
  File "django_saml/views.py", line 122, in saml_acs
    url = saml_auth.redirect_to(req['post_data']['RelayState'])
  File "onelogin/saml2/auth.py", line 214, in redirect_to
    return OneLogin_Saml2_Utils.redirect(url, parameters, request_data=self.__request_data)
  File "onelogin/saml2/utils.py", line 216, in redirect
    OneLogin_Saml2_Error.REDIRECT_INVALID_URL

Thank you.

[voyc-jean]

To support this:
Section 4.1.5 of the OASIS SAML Standard states:

Of special mention is that the identity provider MAY include a binding-specific "RelayState" parameter that
indicates, based on mutual agreement with the service provider, how to handle subsequent interactions
with the user agent. This MAY be the URL of a resource at the service provider. The service provider
SHOULD be prepared to handle unsolicited responses by designating a default location to send the user
agent subsequent to processing a response successfully.

[ngearhart]

I apologize for the lack of response to this issue. Unfortunately, this is currently outside of the scope of work I can handle. Others are welcome to implement this and open a pull request.

[melvyn-apryl]

PR added.