After one hour dealing with the error `ActiveModel::MassAssignmentSecurity::Error` and have to set attr_accessible :blabla, :as => :admin everywhere. I think we should allow user can configure all options to pass to `assign_attributes`.

So in RailsAdmin initializer, we just simply do this:
```
config.attr_accessible_role do
  { without_protection: true }
end
```
And we can still keep `attr_accessible :blabla, :as => :admin` where we want to avoid security threat. Make sense?