Fix rails 6 content type deprecation warnings

What does this MR do?

I had enough of seeing:

DEPRECATION WARNING: Rails 6.1 will return Content-Type header without modification. If you want just the MIME type, please use `#media_type` instead.

in my rspec outputs 😄

Time to do something about it and just follow the suggestion: use #media_type

Screenshots

Example running $ bundle exec rspec ee/spec/requests/api/nuget_packages_spec.rb

Before this MR:

After this MR:

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team