### Describe the issue

Testing in bitcoin 0.13.0, calling `bitcoin-cli listsinceblock x` is outputting all double-spent transactions that are in the wallet, even those that were double-spent well and truly before x.
### Is the issue reproducible?

Yes
#### List steps to reproduce below:
1.   Create a double-spend send to your wallet (easiest using bip125)
2.   Wait until one confirms, and then wait a few more blocks
3.   Run `bitcoin-cli listsinceblock $(bitcoin-cli getbestblockhash)` and it will output the double spent transaction
### Expected behavior

The double-spent transaction that was conflicted before the block hash should be filtered

{
    return "    \"confirmations\": n,                        (numeric) The number of confirmations for the transaction. Negative confirmations means the\n"
           "                                                       transaction conflicted that many blocks ago.\n"
           "    \"generated\": xxx,                          (bool) Only present if transaction only input is a coinbase one.\n"
           "    \"trusted\": xxx,                            (bool) Only present if we consider transaction to be trusted and so safe to spend from.\n"
           "    \"blockhash\": \"hashvalue\",                  (string) The block hash containing the transaction.\n"
           "    \"blockindex\": n,                           (numeric) The index of the transaction in the block that includes it.\n"
           "    \"blocktime\": xxx,                          (numeric) The block time in seconds since epoch (1 Jan 1970 GMT).\n"
           "    \"txid\": \"transactionid\",                   (string) The transaction id.\n"
           "    \"walletconflicts\": [                       (array) Conflicting transaction ids.\n"
           "      \"txid\",                                  (string) The transaction id.\n"
           "      ...\n"
           "    ],\n"
           "    \"time\": xxx,                               (numeric) The transaction time in seconds since epoch (midnight Jan 1 1970 GMT).\n"
           "    \"timereceived\": xxx,                       (numeric) The time received in seconds since epoch (midnight Jan 1 1970 GMT).\n"
           "    \"comment\": \"...\",                          (string) If a comment is associated with the transaction, only present if not empty.\n"
           "    \"bip125-replaceable\": \"yes|no|unknown\",    (string) Whether this transaction could be replaced due to BIP125 (replace-by-fee);\n"
           "                                                     may be unknown for unconfirmed transactions not in the mempool\n";
}
UniValue listtransactions(const JSONRPCRequest& request)