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.
[ci skip] Add "Low-Level Caching" part to "Caching With Rails"
Merge pull request #14285 from sikachu/master-revert-render-body-remove-content-type

Do note remove `Content-Type` when `render :body`
Merge pull request #14257 from killthekitten/patch-1

Add "Low-Level Caching" part to "Caching With Rails" guide