account_tx with forward true is skipping transactions

[ildaruz]

Issue Description

if I use account_tx ws query with forward true subsequent request with marker could skip 1 transaction I'm connecting to wss://s2.ripple.com

Steps to Reproduce

request 1:

{
    "id": 1,
    "command": "account_tx",
    "account": "rf1NrYAsv92UPDd8nyCG4A3bez7dhYE61r",
    "ledger_index_min": 77384137,
    "forward": true,
    "limit": 5
}

response will be with:

TransactionIndexes: 58, 59, 60, 61, 62
"marker": {
    "ledger": 77384137,
    "seq": 63
}

request 2 with marker from previous response:

{
    "id": 2,
    "command": "account_tx",
    "account": "rf1NrYAsv92UPDd8nyCG4A3bez7dhYE61r",
    "limit": 5,
    "forward": true,
    "marker": {
        "ledger": 77384137,
        "seq": 63
    }
}

response will be with:

TransactionIndexes: 64, 65, 66, 67, 68
"marker": {
    "ledger": 77384137,
    "seq": 69
}

transaction with index 63 is skipped

Expected Result

In correct response transaction with index 63 has to be included I can get expected result from wss://xrplcluster.com, but not from s2.ripple.com

TransactionIndexes: 63, 64, 65, 66, 67
"marker": {
    "ledger": 77384137,
    "seq": 68
}