`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 3047376.