Don't override followers_count if already defined

[marckohlbrugge]

I'm using a counter cache to set followers_count on my followable model. Unfortunately the gem seems to override my attribute getter with its own method. So calling record.followers_count still calls the gem's method and executes an unnecessary database query.

My current workaround is to once again override the followers_count method on the followable model and simply return the self[:followers_count] value. This seems like a bit of a hack though.

My proposed solution is for the gem's followers_count method to only get defined if there's no such method yet.

What do you think? I'd be happy to make a PR if you agree with this approach.