Proposed fix to issue #180

Proposed fix to https://github.com/thinktecture/Thinktecture.IdentityServer.v2/issues/180

Forcing all realms to lower-case in the STS cleanup tracking cookie will end up with the cleanup failing for those RPs which have mixed cases.
Merge pull request #181 from nuggetboy/patch-1

Proposed fix to suspected case issue with SignInSessionsManager