fix LDAP authentication with Rails 2.3
fix LDAP authentication with Rails 2.3

In Rails 2.3 ActionDispatch::Session::AbstractStore::SessionHash is not as
smart about lazy loading as in Rails 3, e.g. it doesn't override delete to
make sure that everything's loaded, so in Rails 2.3 creds was always nil.

thomasmillergb/Year2-ruby-software-hut,ngpestelos/omniauth,intridea/omniauth,poznyakandrey/omniauth,nbrustein/omniauth,likeberi/omniauth,mihar/omniauth-timekiwi,ayunav/omniauth,omniauth/omniauth,twalpole/omniauth,github/omniauth,sideci-sample/sideci-sample-omniauth,thomasmillergb/Year2-ruby-software-hut,yui-knk/omniauth,toyokazu/omniauth_shibboleth,dougal/omniauth,fxposter/omniauth