Preserve XSD dateTime and dateTimeStamp lexical representations.

[timothee-haudebourg]

We signing/verifying a VC, ssi will interpret the XSD date/time data and use the canonical representation when serializing to N-quads (in the case of RDF-based linked-data cryptosuites). This behavior is most likely not wanted and prevents interoperability with other signers/verifiers. Instead, we want to preserve the lexical representation we are given, even if it is not the canonical representation.