Allow middleware that extends ActionDispatch::Session::AbstractSecureStore

[ajkerr]

After upgrading to version 0.11.4 of redis-session-store, our application started failing to initialize due to the middleware check in rails_admin.

The failure is due to the security fix in redis-session-store that changes the middleware to inherit from ActionDispatch::Session::AbstractSecureStore instead of ActionDispatch::Session::AbstractStore.

This PR simply changes the check in engine.rb to also allow middlewares that inherit from ActionDispatch::Session::AbstractSecureStore.

Would love to get this in the next 3.x beta.

[coveralls]

Pull Request Test Coverage Report for Build 1772961639

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.03%) to 95.635%

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Files with Coverage Reduction	New Missed Lines	%
app/controllers/rails_admin/application_controller.rb	1	97.78%

Totals
Change from base Build 1765294698:	-0.03%
Covered Lines:	3637
Relevant Lines:	3803

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💛 - Coveralls

[mshibuya]

Makes sense, thanks!