Fix for MongoDB and Missing "/" when redirecting

I am using this plugin with a MongoDB backend and had a problem with a cast to string.
The fix was easier than expected, and I think it won-t break any other client.

Besides, I fix the slash problem using the code suggested in https://github.com/splix/grails-spring-security-facebook/issues/19 by noyzilla. (so far, it is working on my project)