Use resource object instead of the possible non existent @user instance ...

I was getting an undefined method `apply_omniauth' for nil:NilClass, I'm using spree_devise_auth, I suppose you're overriding the devise build_resource method, but looking at the devise code I found that the @user instance var is the result of the build_resource method, so, you can't use it inside the build_resource method, instead I used the resource object provide by devise