Issue where BigInt.toByteArrayUnsigned returns 31 bytes

Here is an example root key:
tprv8ZgxMBicQKsPetSeaqxJJLxz6rRvxz2fuTBKt4ntiqRjTAdmqkqdFKDCX9X9dgPLcEXELNSADqXkDzHs77WJcP6wDvQ5XcX7k5UxBiAZTzH

For some reason BigInt.toByteArrayUnsigned returns byte array without the leading 0 byte, inserting this into the byte array in the case of the length being 31 seems to fix this
