Allow single type

Fix w3c#97

Author: clehner

test/vc-data-model-1.0/input/example-3-bad-cardinality.jsonld

@@ -4,7 +4,7 @@
     "https://www.w3.org/2018/credentials/examples/v1"
   ],
   "id": "http://example.edu/credentials/3732",
-  "type": "VerifiableCredential",
+  "type": [],
   "issuer": "https://example.edu/issuers/14",
   "issuanceDate": "2010-01-01T19:23:24Z",
   "credentialSubject": {