rest: invalid deposit class detection when missing pid_value view argument causes missing action "links"

[slint]

On POST calls, since request.view_args['pid_value'] is not part of the url (/api/deposit/depositions), the detected deposit class always falls back to invenio_deposit.api.Deposit, causing any additional action links added using the invenio_deposit.urils.mark_as_action decorator to be skipped.

Perhaps adding a pid_type -> deposit_class mapping as part of a current_records_rest.default_pid_classes might help to determine the desired default:

deposit_cls = current_records_rest.default_pid_classes.get(pid.pid_type, Deposit)
if 'pid_value' in request.view_args:
    deposit_cls = request.view_args['pid_value'].data[1].__class__

Update: This issue leads to incorrect links generation which is extremely problematic when there are additional actions implemented in a deposit class, and responses have to be consumed by a Javascript UI application. Example:

0. (assume we have a custom deposit class with a MyDeposit.custom_action)
1. Client does POST /api/deposit/depositions, gets back depid: 1234 and links with missing custom_action link.
2. Client wants to perform custom_action, but link is missing...
3. Client "reloads" the page (i.e. refetches /api/deposit/depositions/1234), which now contains the missing custom_action link.