When switching to or creating a new auth context, all previously stored contexts are lcased

If you `doctl auth init --context Dog` it is initially stored with case sensitivity. However, If you switch to another context `doctl auth switch default` or `doctl auth init --context Cat` all the `auth-contexts` stored in _~/.config/doctl/config.yaml_ become lcased.

The problem is when you try to switch back to `Dog` it is no longer authorized because the stored context name has changed to `dog`.

**doctl version 1.13.0-release**