Fix default relay state bug

[lgarvey]

If not defined the relaystate was always defaulting to / despite settings.LOGIN_REDIRECT_URL being set.

[lgarvey]

This PR fixes the issue I face - e.g. being able to set a redirect URL but as a result this code will never get hit:

https://github.com/peppelinux/djangosaml2/pull/253/files#diff-14e9a5c86c429d4f469f5e22f7c2c4b239e52569a920f15c58513f2d2a216332R438-R439

Is it important to log that RelayState was not provided?

Either I could remove the check/logging statement (lines 438/439) or restructure it so that it is correctly aware if the incoming request has no relay state.

[peppelinux]

Please rebase to Dev branch.
Sounds good and I'd also appreciate a unittest.
Probably this PR Will need some review but yes, looks good

[lgarvey]

@peppelinux This still needs a test and clearing up the logging section. I'll raise another PR! :)

PS. UniAuth looks interesting.

[peppelinux]

I'll wait for a PR with the test in the meanwhile.
uniAuth is quite monolitich and it would be refactored to be a django app but, at the same time, I think that a monolitich IdP is the scope of uniAuth... so I feel quite torn about this.

The next release of djangosaml2 will take some weeks, so, time is not a problem.