message: listspendzerocoins output: change serial to hex
diff: @@ -2972,7 +2972,7 @@ UniValue listspendzerocoins(const UniValue &params, bool fHelp) {
  " \"txid\": \"transactionid\", (string) The transaction hash\n"
  " \"denomination\": d, (numeric) Denomination\n"
  " \"spendid\": id, (numeric) Spend group id\n"
  - " \"version\": \"v\", (string) Spend version (1.0, 1,5 or 2.0)\n"
  + " \"version\": \"v\", (string) Spend version (1.0, 1.5 or 2.0)\n"
  " \"modversion\": mv, (numeric) Modulus version (1 or 2)\n"
  " \"serial\": \"s\", (string) Serial number of the coin\n"
  " \"abandoned\": xxx, (bool) True if the transaction was already abandoned\n"
  @@ -3021,7 +3021,7 @@ UniValue listspendzerocoins(const UniValue &params, bool fHelp) {
  entry.push_back(Pair("modversion", fModulusV2 ? 2 : 1));
  entry.push_back(Pair("version", spendVersion==ZEROCOIN_TX_VERSION_1 ? "1.0" :
  (spendVersion==ZEROCOIN_TX_VERSION_1_5 ? "1.5" : "2.0")));
  - entry.push_back(Pair("serial", spend.getCoinSerialNumber().ToString()));
  + entry.push_back(Pair("serial", spend.getCoinSerialNumber().GetHex()));
  ret.push_back(entry);
  