Add a default constructor so Spring can instantiate + DI

The following commit adds a constructor for `DefaultUserInfoUserDetailsService`:
https://github.com/mitreid-connect/OpenID-Connect-Java-Spring-Server/commit/a7f2e605fad3e85e2949368ea27d960251941853

... 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 broken the build).