The default should be verificationMethodSig and not UNKNOWN_VERIFICATION_METHOD.

[richtera]

When reading JSONURL or VerifiableURI we should return unknown verification methods as just their signature. Without doing this the library will not support custom verification methods until they are officially ratified inside of the library. During read Verification.method should be the name if it's known or the signature if not. The data should be forwarded according to the length if it's VerifiableURI or 32 bytes if it's JSONURL.

The default should be verificationMethodSig and not UNKNOWN_VERIFICATION_METHOD.

Originally posted by @richtera in #365 (comment)