Close the original body passed to Rack::ETag

The Rack spec states:

> If the Body responds to `close`, it will be called after iteration. If
> the body is replaced by a middleware after action, the original body
> must be closed first, if it responds to close.

After Rack::ETag replaces the response body, close the original body.

Signed-off-by: David Celis <me@davidcel.is>