der: trim leading zeros from integers

[jablko]

Before

const INTEGER = asn1.define('INTEGER', function() {
  this.int();
});
console.log(INTEGER.encode(Buffer.from('0001', 'hex'))); // <Buffer 02 02 00 01>

After

console.log(INTEGER.encode(Buffer.from('0001', 'hex'))); // <Buffer 02 01 01>