[DURACOM-326] fix possible issue on missing value for eperson patch

[FrancescoMolinaro]

References

Fixes #4287

Description

Improve patch for EPerson to allow "add" operation.

Instructions for Reviewers

Inserted "add" operation as possible for EPerson modification.
Implemented test to ensure presence of "add" operation, created mock.

To reproduce the bug the easiest way is to delete one of the metadata on the DB, e.g. lastname, for an EPerson, and then try to add that metadata via UI. (Step by step at #4287).

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[tdonohue]

@FrancescoMolinaro : Thanks for this PR. I gave this a test today, and it nearly works, but it appears the EPerson changes are no longer saving.

Here's what I've tried with this PR in place.

1. First, I'm running in production mode (npm run build:prod; npm run serve:ssr)
2. Go in the Database, and delete the Last Name of a specific EPerson.
3. Now, login as an Admin from the UI
4. Find that EPerson account in the "Access Control" -> "People" area. Verify that it has no Last Name
5. Now, edit that EPerson. Add a new Last Name. Click "Save". A confirmation of the new name appears (no errors occur).
6. Find that EPerson again. It still has no Last Name.
7. Refresh the page, It still has no Last Name.
8. Check in the database, and it still has no Last Name in the database either.

Is there something missing from this PR on the backend? It appears to me that the adding a new value to an EPerson isn't working properly.

[FrancescoMolinaro]

Hi @tdonohue, thanks for the feedback, I tried again following the same steps you did but unfortunately I can't replicate the issue and I can see the the metadata in the DB and in the UI once added.
In my testing environment I am using the main branch for the rest without any modification to the code, may I please ask you to check the network request made once the form is saved?
We should have an "add" operation sent to the backend; the behavior described from your testing seems to be the same we had before the fix, were a "replace" operation was sent for a missing metadata, therefore failing to update the EPerson even though a success message was displayed.

I will be looking forward to your feedback.