Validator does not test for @context in v3 manifests

[benwbrum]

The validator does not show an error for a v3 collection manifest missing a @context element. By contrast, the spec states that, The top level resource in the response must have the @context property

I just tested this manifest without error: https://lbiiif.riksarkivet.se/collection/arkiv/lZkyF37hGIL2wcbGB1Vju6

Current manifest contents (in case the manifest is fixed before this issue is addressed):

{
  "id": "https://lbiiif.riksarkivet.se/collection/arkiv/lZkyF37hGIL2wcbGB1Vju6",
  "type": "Collection",
  "label": {"sv":["D VII Utvandrarrulla"]},
  "summary": {
    "sv": [
      "Referenskod: SE/SSA/0021/01/D VII",
      "Arkivinstitution: ",
      "Datering: "
    ]
  },
  "items": [
    {
      "id": "https://lbiiif.riksarkivet.se/arkis!A0069178/manifest",
      "type": "Manifest",
      "label": {"sv":["1 (1869-1886) - A0069178"]}
    },
    {
      "id": "https://lbiiif.riksarkivet.se/arkis!A0069179/manifest",
      "type": "Manifest",
      "label": {"sv":["2 (1886-1888) - A0069179"]}
    },
    {
      "id": "https://lbiiif.riksarkivet.se/arkis!A0069180/manifest",
      "type": "Manifest",
      "label": {"sv":["3 (1888-1904) - A0069180 - Bunt."]}
    }
  ]
}