gitaly-lfs-smudge: Fix missing close for HTTP body

The gitaly-lfs-smudge command is a smudge filter for Git which will replace contents of LFS pointers with the actual LFS object's contents. To do so, we need to request the object's contents from Rails via an HTTP request. The tests exercising this code all of a sudden started failing due to leaking Goroutines, where the leak happens in the HTTP handling code. And sure enough: we never close the http.Response body, which may likely be the root cause here.

Fix this by always closing the body. While I have no idea why leaks started to happen just now, chances are high that this fixes the new flake.