[Rails5] Fix `undefined method 'downcase'` errors in requests specs

What does this MR do?

In Rails 5.0 headers are no longer inherited from Hash.

This MR fixes errors in requests specs like:

1) API::V3::Builds GET /projects/:id/builds/:build_id/artifacts job with artifacts when artifacts are stored locally authorized user returns specific job artifacts
    Failure/Error: expect(response.headers).to include(download_headers)

    NoMethodError:
      undefined method `downcase' for #<Hash:0x000055e8bccc23b0>
    # ./spec/requests/api/v3/builds_spec.rb:235:in `block (6 levels) in <top (required)>'

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

No.

Why was this MR needed?

Migration to Rails 5.0.

Screenshots (if relevant)

No.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

#14286 (closed) and !12841 (closed)