rest: incorrect disk quota update when overwriting files

[mdonadoni]

How to reproduce:

1. Enable disk quota accounting

--- a/helm/reana/values.yaml
+++ b/helm/reana/values.yaml
@@ -142,4 +142,4 @@ quota:
   workflow_termination_update_policy: "{{ .Values.quota.termination_update_policy }}"
   # backward compatibility
   disk_update: "0 3 * * *"  # everyday at 3am
-  termination_update_policy: ""
+  termination_update_policy: "disk,cpu"

2. Create a new demo workflow: reana-client create -f reana.yaml -w helloworld
3. Upload the workflow's files multiple times: reana-client upload -w helloworld

Each time the files are uploaded, the user and workflow disk usages are increased by the size of the uploaded files. This happens even when, from the second upload onwards, the files are being overwritten. In this case, the disk usage should not increase, given that the disk space used by the workflow stays the same.