Implement validation of VCard

Closes https://code.vereign.com/code/signing-service-agent/issues/32

javascript/src/utilities/signingUtilities.js

@@ -418,7 +418,7 @@ function createCertificate(certData, issuerData = null) {
         serialNumberView = new Uint8Array(certData.serialNumber);
       } else if (certData.serialNumber instanceof Uint8Array) {
         serialNumberView = certData.serialNumber;
-      } else if (certData.serialNumber instanceof String) {
+      } else if (typeof certData.serialNumber === "string") {
         try {
           serialNumberView = new Uint8Array(hexStringToBytes(certData.serialNumber));
         } catch (ignore) {