Fix inheritance of parent_controller when controllers were eagerly loaded

[jalyna]

Follow up on #2790

5bd9805 fixes the inheritance of the RailsAdmin::ApplicationController, nevertheless RailsAdmin::MainController.superclass.superclass is ActionController::Base instead of the configured ::ApplicationController, when accessing the main controller first (happens with eager loading).

Before the fix:

  1) RailsAdmin::Config.parent_controller= if RailsAdmin::ApplicationController is already loaded can be changed
     Failure/Error: expect(RailsAdmin::MainController.superclass.superclass).to eq ApplicationController

       expected: ApplicationController
            got: ActionController::Base

       (compared using ==)

       Diff:
       @@ -1 +1 @@
       -ApplicationController
       +ActionController::Base

[coveralls]

Pull Request Test Coverage Report for Build 1739656647

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 95.829%

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Totals
Change from base Build 1736071775:	0%
Covered Lines:	7053
Relevant Lines:	7360

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💛 - Coveralls

[mshibuya]

Awesome, thanks!