fixed frame on type `@json`

[pasquale95]

Fixed the frame for the case of framing over a @json attribute for the issue #142. The previous frame expanded incorrectly as:

[
  {
    "http://example.org/vocab#info": [
      {
        "@type": "@json",
        "@value": {
          "@value": {},
          "@type": "@json"
        }
      }
    ]
  }
]

This frame expands instead as:

[
  {
    "http://example.org/vocab#info": [
      {
        "@type": "@json",
        "@value": {}
      }
    ]
  }
]

which should be the expected expansion.

[w3cbot]

gkellogg marked as non substantive for IPR from ash-nazg.