make the integer positive

javascript/src/utilities/signingUtilities.js

@@ -222,6 +222,7 @@ function createCertificate(certData, issuerData = null) {
     const serialNumberBuffer = new ArrayBuffer(20);
     const serialNumberView = new Uint8Array(serialNumberBuffer);
     pkijs.getRandomValues(serialNumberView);
+    serialNumberView[0] &= 0x7f;
     // noinspection JSUnresolvedFunction
     certificate.serialNumber = new asn1js.Integer({
       valueHex: serialNumberView