Ensure factory_bot only loads after initialization

Fixes #336
Alternate solution to #343

The initialization process looks like this:

Rails runs all of the initializers
1. Run the ["factory_bot.set_factory_paths"][set_factory_paths]
initializer
2. Run the ["factory_bot.register_reloader"][register_reloader]
initializer, which sets up a [prepare callback][]
3. Run the [`:run_prepare_callbacks`][] initializer
4. This triggers the factory_bot [prepare callback][], which causes
factory\_bot to [reload][]

Rails runs `after_initialize` callbacks
1. [I18n initializes]
2. factory\_bot [reloads again][] as described in #334

The double reloading of factory_bot in this initialization is not ideal,
but also shouldn't generally cause any problems on its own.

The problems people are having in #336 come from the fact that
I18n gets set up in an `after_initialize` callback, but factory_bot gets
reloaded before the `after_initialize` callbacks are triggered.
If the `FactoryBot.define` block references any code that uses I18n
translations as it loads, that code will raise an error (references
inside other factory_bot methods, or code that uses I18n translations
inside of methods still works fine, since the whole Rails initialization
process would be complete by the time any of that code runs).

This commit changes step 4 above to avoid reloading factory_bot before
the application has initialized.

[set_factory_paths]: 3815aae2b9/lib/factory_bot_rails/railtie.rb (L17-L19)
[register_reloader]: 3815aae2b9/lib/factory_bot_rails/railtie.rb (L21-L23)
[prepare callback]: https://github.com/thoughtbot/factory_bot_rails/blob/master/lib/factory_bot_rails/reloader.rb#L34-L36
[`:run_prepare_callbacks`]: https://github.com/rails/rails/blob/5-2-stable/railties/lib/rails/application/finisher.rb#L62-L64
[reload]: https://github.com/thoughtbot/factory_bot_rails/blob/master/lib/factory_bot_rails/reloader.rb#L24-L26
[I18n initializes]: 13e2102517/activesupport/lib/active_support/i18n_railtie.rb (L16-L20)
[reloads again]: https://github.com/thoughtbot/factory_bot_rails/blob/master/lib/factory_bot_rails/railtie.rb#L25-L27

Co-authored-by: Danny Garcia <dannygarcia.me@gmail.com>

features/reloading.feature

@@ -63,3 +63,45 @@ Feature:
     And I run `spring stop` with a clean environment
     Then the output should contain "1 runs, 1 assertions"
     And the output should not contain "Failure:"
+
+Scenario: Initializing the reloader with I18n support
+    When I successfully run `bundle exec rails new testapp -m ../../features/support/rails_template`
+    And I cd to "testapp"
+    And I add "factory_bot_rails" from this project as a dependency
+    And I add "test-unit" as a dependency
+    And I run `bundle install` with a clean environment
+    And I run `bundle exec rake db:migrate` with a clean environment
+    And I write to "app/models/user.rb" with:
+      """
+      class User
+        TRANSLATION = I18n.translate("translation_key")
+      end
+      """
+    And I write to "config/locales/en.yml" with:
+      """
+        en:
+          translation_key: "translation_value"
+      """
+    And I write to "test/factories.rb" with:
+      """
+      FactoryBot.define do
+        factory :user do
+          User::TRANSLATION
+        end
+      end
+      """
+    And I write to "test/unit/user_test.rb" with:
+      """
+      require 'test_helper'
+
+      class UserTest < ActiveSupport::TestCase
+        test "use factory" do
+          user = FactoryBot.build(:user)
+
+          assert_equal "translation_value", User::TRANSLATION
+        end
+      end
+      """
+    And I run `bundle exec rake test` with a clean environment
+    Then the output should contain "1 runs, 1 assertions"
+    And the output should not contain "Failure:"

lib/factory_bot_rails/reloader.rb

@@ -31,9 +31,13 @@ module FactoryBotRails
     end
 
     def register_reloader(reloader)
+      closed_over_app = app
+
       config.to_prepare do
+        if closed_over_app.initialized?
           reloader.execute
         end
+      end
 
       app.reloaders << reloader
     end