This was discovered by @xpkoala while performing a load test for the
calendar backoff feature.
Some enroll requests were failing due to enrolling hosts too fast (`-var
loadtest_containers` from `0` to `40` at once), and osquery-perf had a
bug in the enroll request where the `bytes.Buffer` was being incorrectly
reused thus sending an empty body on the enroll retries, getting 400s
from Fleet due to `Expected JSON Body`:
```
2024/04/11 18:57:49 request failed: 400
```
Lucas Manuel Rodriguez
a86da9f74b