Fix obsoleted method URI.unescape in activesupport/test

[vbrazo]

Summary

When running bundle exec rake, I was getting this warning:
/Users/someone/rails/activesupport/test/core_ext/object/to_query_test.rb:91:in 'test_hash_not_sorted_lexicographically_for_nested_structure': warning: URI.unescape is obsolete

I think this test should be using a method that isn't obsolete.

[rails-bot]

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[kamipo]

Thanks!