Invalid case conversion of words containing digits

Strings such as `k8s` and `g2a` aren't correctly converted between UPPER_CASE and camelCase. Currently conversion algorithm treats boundaries between digits and letters as word boundaries. This leads to confusing conversions like:

```
k8sHost → K_8_S_HOST
K8S_HOST → k8SHost
```

Expected conversions:

```
k8sHost → K8S_HOST
K8S_HOST → k8sHost
```