An Ed25519Signature2018 can be created without a verificationMethod

[mattcollier]

Minimal runnable example: https://repl.it/repls/JollyIrritatingResearch

The proof produced in the example is not valid:

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://w3id.org/citizenship/v1"
  ],
  "id": "https://issuer.uscis.gov/credentials/fe874db0-37a3-4ddc-b892-875dc5e36815",
  "type": [
    "VerifiableCredential",
    "PermanentResidentCard"
  ],
  "issuer": {
    "id": "did:v1:test:nym:z6Mkq4uyWFYWP9rPrrKhJnbU7jBDBc2p8gXJHyQ26L6uS8uD"
  },
  "issuanceDate": "2020-01-01T00:00:00.000Z",
  "expirationDate": "2030-01-01T00:00:00.000Z",
  "name": "Permanent Resident Card",
  "description": "Permanent Resident Card",
  "credentialSubject": {
    "id": "did:v1:test:nym:z6Mkf5t55Qb2tg5UqqEsukL3XyK67r6pKEAYQ3Rg9DhmoZmG",
    "type": [
      "Person",
      "PermanentResident"
    ],
    "givenName": "Louis",
    "familyName": "Pasteur",
    "gender": "Male",
    "residentSince": "2015-01-01T00:00:00.000Z",
    "lprCategory": "C09",
    "lprNumber": "999-999-999",
    "birthCountry": "Mexico",
    "birthDate": "1958-07-17T00:00:00.000Z"
  },
  "proof": {
    "type": "Ed25519Signature2018",
    "created": "2020-02-27T16:47:57Z",
    "jws": "eyJhbGciOiJFZERTQSIsImI2NCI6ZmFsc2UsImNyaXQiOlsiYjY0Il19..PLizo6Cl1L4H1c-ySBubANkEGqS7TydS7sz4Kv4uT2eyECg7HcSCykDCu3tZ3eHEoLDtnkcSvInbqN7h2P36DQ",
    "proofPurpose": "assertionMethod"
  }
}