Rails5 fix expected: 1 time with arguments: (97, anything, {"squash"=>false}) received: 0 times

What does this MR do?

Fix spec/controllers/projects/merge_requests_controller_spec.rb:350 for Rails 5. Rails 5 performed the test with "squash"=>"false" as param instead of "squash"=>false. Fix(as: :json) found here: https://github.com/rails/rails/issues/26075#issuecomment-374826541.

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

The spec was failing with a different error on CI as locally. I didn't see the spec anymore in the failures for the pipeline, so I guess its fixed.

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
• 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
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #48104 (closed)