Do note remove `Content-Type` when `render :body`

`render :body` should just not set the `Content-Type` header. By
removing the header, it breaks the compatibility with other parts.

After this commit, `render :body` will returns `text/html` content type,
sets by default from `ActionDispatch::Response`, and it will preserve
the overridden content type if you override it.

Fixes #14197, #14238

This partially reverts commit 3047376870d4a7adc7ff15c3cb4852e073c8f1da.
Fix guide about `config.active_record.default_timezone` [ci skip]

Now the initial value of ActiveRecord.default_timezone is always :utc
Merge pull request #14296 from joker1007/fix_rails_guides_about_active_record_default_timezone

Fix guide about `config.active_record.default_timezone` [ci skip]
Merge pull request #14285 from sikachu/master-revert-render-body-remove-content-type

Do note remove `Content-Type` when `render :body`