Remove deprecation warning in rails 7

[existentialmutt]

Rails 7 gives the following deprecation warning when running rspec-apidoc:

Rails 7.1 will return Content-Type header without modification. If you want just the MIME type, please use `#media_type` instead.

This PR updates to the new #media_type method which has the same behavior as older versions of content_type

[stas]

@existentialmutt thanks for the PR. Do you know if this is backwards-compatible with Rails 6/5 or we need to bump the minimum dependency version as well?

[existentialmutt]

Good call, @stas . Let me check on that.

[existentialmutt]

Looks like this is compatible with Rails 6 but not 5.