Reject any field updates not allowed on POST

    This is done on fields with `reject_update=True`.

def _from_rest_reject_update(model):
    """ Reject any field updates not allowed on POST

    This is done on fields with `reject_update=True`.
    """

    dirty = model.dirty_fields
    fields = model.get_fields_by_prop('reject_update', True)
    reject = []

    for field in fields:
        if field in dirty:
            reject.append(field)

    if reject:
        mod_fail('These fields cannot be updated: %s' % ', '.join(reject))