fix: stringify for normalization json-pointer for JSON string data

[namhoang1604]

When createProof using the normalization json-pointer with the document content the value is JSON string, that will raise an error.
Example document:

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://www.w3.org/2018/credentials/examples/v1"
  ],
  "type": [
    "VerifiableCredential",
    "UniversityDegreeCredential"
  ],
  "credentialSubject": {
    "name": "Jane Smith",
    "degree": {
      "type": "BachelorDegree",
      "name": "Bachelor of Science and Arts",
      "degreeSchool": "{\"name\": \"Example University\"}"
    }
  },
  "issuer": {
    "id": "did:web:example.com"
  }
}