Allow Spring to instantiate DefaultUserInfoUserDetailsService via DI

https://github.com/mitreid-connect/OpenID-Connect-Java-Spring-Server/commit/a7f2e605fad3e85e2949368ea27d960251941853 adds a new constructor for `DefaultUserInfoUserDetailsService` ...


... and the combination of **having *some* constructor** and not having an empty one prevents Spring from instantiating the bean automatically on deployment.

Great to see that unit tests are a-coming  (and ironic, I guess, that this one broke the build).