Currently, calling `head` to set the response code silently removes a
previously set response body [0]. This can lead to unnecessary and
potentially expensive rendering, as shown in the following snippet:

```ruby
def index
  render json: User.all.as_json # expensive rendering
  head :ok # Overwrites `response_body` to ``
end
```

This behavior is inconsistent with calling `render` twice or calling
`redirect_to` after `render`: both of which raise a `DoubleRenderError`
[1, 2].

Since this change could break existing endpoints when upgrading Rails,
it might make sense to introduce a configuration option would allow
developers to choose whether this scenario raises an exception or
a warning.

[0] https://github.com/rails/rails/blob/b721c62de11c2c29fa2b8ef51b00d95dce163afc/actionpack/lib/action_controller/metal/head.rb#L50
[1] https://github.com/rails/rails/blob/b721c62de11c2c29fa2b8ef51b00d95dce163afc/actionpack/lib/action_controller/metal/redirecting.rb#L103-L105
[2] https://github.com/rails/rails/blob/b721c62de11c2c29fa2b8ef51b00d95dce163afc/actionpack/lib/action_controller/metal/rendering.rb#L171-L174