Rails5 fix NoMethodError: undefined method `join' for "":String

What does this MR do?

Use string as hash key in the spec because that is what application.attributes uses for keys. Previous application_attributes had a "scopes" and :scopes key:

{"id"=>nil,
 "name"=>"OAuth App 1",
 "uid"=>"15a37303a3747cf98330b3161630316008edd0d147f9f4c8d8f173f7eb4fa386",
 "secret"=>nil,
 "redirect_uri"=>"http://example1.org",
 "scopes"=>"",
 "created_at"=>nil,
 "updated_at"=>nil,
 "owner_id"=>2,
 "owner_type"=>"User",
 "trusted"=>false,
 :scopes=>[]}

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

rails5 upgrade

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• EE specific content should be in the top level /ee folder
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #6607 (closed)