Fixed the broken Ruby 2.7 due to missing OpenStruct. #105

Job	Run time
Test the gem (Ruby 2.5, Rails 5.2)	1m 6s
Test the gem (Ruby 2.5, Rails 6.0)	57s
Test the gem (Ruby 2.5, Rails 6.1)	54s
Test the gem (Ruby 2.6, Rails 5.2)	58s
Test the gem (Ruby 2.6, Rails 6.0)	58s
Test the gem (Ruby 2.6, Rails 6.1)	1m 0s
Test the gem (Ruby 2.7, Rails 5.2)	1m 34s
Test the gem (Ruby 2.7, Rails 6.0)	1m 7s
Test the gem (Ruby 2.7, Rails 6.1)	1m 19s
Test the gem (Ruby 2.7, Rails 7.0)	1m 1s
Test the gem (Ruby 3.0, Rails 5.2)	57s
Test the gem (Ruby 3.0, Rails 6.0)	56s
Test the gem (Ruby 3.0, Rails 6.1)	56s
Test the gem (Ruby 3.0, Rails 7.0)	1m 13s
Test the gem (Ruby 3.1, Rails 5.2)	1m 1s
Test the gem (Ruby 3.1, Rails 6.0)	48s
Test the gem (Ruby 3.1, Rails 6.1)	48s
Test the gem (Ruby 3.1, Rails 7.0)	46s
Trigger the documentation upload	18s
	18m 37s

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