Bugfixes for integration

[reinkrul]

When integrating in Nuts node, I came across a few bugs:

• Typo in JWTCredentialsProofFormat const name
• MarshalJSON() for Verifiable Credentials in JWT format returned the VC as JSON (as if it were JSON-LD), instead of the parsed JWT VC
• JWT VCs contained null/empty values for properties that are specified as JWT claims (issuanceDate, issuer, proof). Moved JWT creation function from Nuts node to this library to create JWT credential without setting null/empty properties.
• Made expirationDate in Verifiable Credential in JWT format optional (now led to a zero-ed time.Time)

[gerardsn]

I don't feel comfortable being the only reviewer on this

[gerardsn]

LGTM. remaining concerns need a rewrite of the library. (VC probably needs to be a map with some public methods for common fields)