Fix nil formats test

What does this MR do?

Fixes setting of action_dispatch.request.formats in a spec.

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

Why was this MR needed?

One of the spec files sets nil value for env varialbe action_dispatch.request.formats. Rather than setting nil value, we can delete the action_dispatch.request.formats key from env (which should be closer to real use case anyway). The reason is that in Rails 5, formats method does not set default array of formats if the env key is already set (even if set to nil).

We don't set this variable anywhere in the code (at least I didn't find it).

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 #47833 (closed)