HED issues are not translated into "ordinary" Issue objects

[happy5214]

hed-validator returns a list of BidsIssue objects, which usually involves the BidsHedIssue subclass. This is intended to be a type extension of the Issue class, but https://github.com/bids-standard/bids-validator/blob/master/bids-validator/validators/hed.js fails to convert the BidsIssue objects to Issue objects to remove any extra fields, like the hedIssue field in BidsHedIssue. This is believed to be the cause of OpenNeuroOrg/openneuro#3162.

Please assign this to me, as I'll fix this shortly.