bars API returns wrong response sometimes

Possibly related to issue [#139](https://github.com/alpacahq/Alpaca-API/issues/139)

The following endpoint returns a non-deterministic response.

https://data.alpaca.markets/v1/bars/minute?symbols=TSLA&limit=1&until=2021-01-25T15:30:00.000Z

Expected behavior: API should always return a single bar whose time is 2021-01-25, 15:29:00 pm.
Actual behavior: Sometimes, (seemingly randomly,) the API will wrongly return the following:

{
"TSLA": [
{
"t": 1611588480,
"o": 887.935,
"h": 887.935,
"l": 885.73,
"c": 886.27,
"v": 3319
}
]
}

which is the bar for 2021-01-25, 15:28:00 pm not 15:29:00 pm.