Bug 37592: (QA follow-up) Add API mapping and definition

Sticking to API guidelines, this adds the creation_date and
modification_date fields to the api definitions and the required
to_api_mappings for those fields to be properly populated.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>

Koha/Booking.pm

@@ -241,7 +241,10 @@ on the API.
 =cut
 
 sub to_api_mapping {
-    return {};
+    return {
+        created_on => "creation_date",
+        updated_on => "modification_date"
+    };
 }
 
 =head3 delete

api/v1/swagger/definitions/booking.yaml

@@ -10,6 +10,10 @@ properties:
   booking_id:
     description: Internal booking identifier
     type: integer
+  creation_date:
+    description: Creation date and time of this booking
+    format: date-time
+    type: string
   end_date:
     description: Start date and time of this booking
     format: date-time
@@ -24,6 +28,10 @@ properties:
     type:
       - object
       - 'null'
+  modification_date:
+    description: Modification date and time of this booking
+    format: date-time
+    type: string
   patron_id:
     description: Internal patron identifier
     type: integer