Bug 34333 added the ability to embed cancellation requests information on the holds endpoint, but the `collectionFormat` attribute got misaligned, making the feature unusable.

This is hard to catch. Steps to reproduce:
1. Open your favourite REST tool
2. Make this request:

GET http://kohadev-intra.myDNSname.org:8081/api/v1/patrons/42/holds
x-koha-embed: cancellation_requested

=> SUCCESS: Probably empty result, but not an error

3. Make this change:

 $ git diff
diff --git a/api/v1/swagger/paths/patrons_holds.yaml b/api/v1/swagger/paths/patrons_holds.yaml
index e8e1335a87d..c70d3359c41 100644
--- a/api/v1/swagger/paths/patrons_holds.yaml
+++ b/api/v1/swagger/paths/patrons_holds.yaml
@@ -24,6 +24,7 @@
           type: string
           enum:
             - cancellation_requested
+            - pickup_library
           collectionFormat: csv
     produces:
       - application/json

4. Restart plack:
   $ ktd --shell
  k$ koha-plack --restart kohadev
5. Repeat 2
=> SUCCESS: Same thing
6. Amend the request like this:

GET http://kohadev-intra.myDNSname.org:8081/api/v1/patrons/42/holds
x-koha-embed: cancellation_requested,pickup_library

=> FAIL: You get this, which is weird because it matches exactly what's in the x-koha-embed `enum`:

{
    "errors": [
        {
            "message": "Not in enum list: cancellation_requested, pickup_library.",
            "path": "/x-koha-embed/0"
        }
    ],
    "status": 400
}


This is related to bug 35614.