should handle namespaced in field/associative

Created by: namiwang

Say we've got some models like

module OneModule
  class OneModel < ApplicationRecord
    belongs_to :another_model, class_name: 'AnotherModel', foreign_id: 'another_model_id'
  end
end

While Rails is ok about it, administrate needs class_name to be OneModule::AnotherModel instead of just AnotherModel to work properly.

Guess the related methods is associated_class_name in field/associative.