Proposed fix to suspected case issue with SignInSessionsManager

I feel weird submitting this as I haven't tested with IdentityServer specifically.  However, in running through my own custom STS I'm building, I noticed that the logic used in `AddEndpoint()` and `WriteCookie()`, specifically the calls to `ToLowerInvariant()`, make the wsignoutcleanup1.0 calls ineffectual when dealing with RP realm URIs with mixed-case.  Example:

- You have a relying party realm `http://someserver/SomeMixedCaseAppPath/blah`
- This relying party gets added to the cleanup-tracking cookie on the STS as `http://someserver/somemixedcaseapppath/blah` with all lower-case characters.
- When the time comes to call `http://someserver/somemixedcaseapppath/blah?wa=wsignoutcleanup1.0` the result is a little green checkmark from the RP, but I can confirm using my custom code that there is no RP signout unless the realm URI matches  case-sensitively.

I'm guessing these calls to `ToLowerInvariant()` are done to ensure no duplicates get added to the tracking cookie, based on the case?

I would submit that the calls to `ToLowerInvariant()` in `AddEndpoint()` and `WriteCookie()` need to be removed and this will fix the issue.  Please see the associated pull request.