[Rails5] Explicitly set request.format for blob_controller

What does this MR do?

In Rails 4.2 if params[:format] is empty, Rails set it to :html.
But since Rails 5.0 the framework now looks for an extension.

E.g. for blob/master/CHANGELOG.md in Rails 4 the format would be :html, but in Rails 5 on it'd be :md.

This MR explicitly sets the :html format for all requests unless :format is set by a client e.g. by JS for XHR requests.

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
• 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 #47921 (closed)