Missing test coverage on ring buffer's wrapping behavior

Tests for the NatSampler don't cover the case where the ring buffer wraps back to the beginning on line 23 - https://github.com/mafintosh/nat-sampler/blob/6ad6c9abdc07a8cb5dfde581df126f869e470560/index.js#L23.

This is the uncovered code. I've written tests for the equivalent Go code in my project and can confirm that the algorithm is correct.

```
} else {
if (this._top === 32) this._top = 0

const oa = this._samples[this._top]
this._samples[this._top++] = a
oa.hits--

const ob = this._samples[this._top]
this._samples[this._top++] = b
ob.hits--
}
```

Start with the wrapping branch in index.js at line 23 and inspect how NatSampler currently tests adjacent behavior. Add coverage for the ring buffer returning to the beginning after reaching the top, then run the existing test suite to confirm the wrapping path is exercised.

Written by the indexing model from the issue text.