Fix CurrentUser MVEL object applying "user:" prefix to name property

Steps to Reproduce:

1. Define Studio project with:
   • custom doctype that includes a multi-valued user/group field (OwnerTester in attached .jar)
   • automation chain and event handler that populates above field with {{@ {CurrentUser.name}

     }} at Empty Document Created

2. In Web UI, create new instance of OwnerTester. Observe default value applied in nuxeo-user-suggestion widget of the Create layout, and create the document.
3. Examine document's JSON export and observe ownertester:owner field - the username is prefixed with the string "user:".
4. Create a new instance of OwnerTester. In the nuxeo-user-suggestion widget, remove the default value and re-select the user. Create the document.
5. Examine document JSON export and observe ownertester:owner field - the username is not prefixed.

Expected behavior: the username pre-populated using CurrentUser object is not prefixed with 'user:'.

Actual behavior: the username pre-populated using CurrentUser object is prefixed with {{'user:' }}and there is no option to disable this.