Bug 29405: Fix date_renewed attribute format in patron spec

This patch just adds the 'date' format string to ensure we are
validating the input/output of the date_renewed field correctly.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>

api/v1/swagger/definitions/patron.json

@@ -144,6 +144,7 @@
     },
     "date_renewed": {
       "type": ["string", "null"],
+      "format": "date",
       "description": "date the patron's card was last renewed"
     },
     "incorrect_address": {