If the current kubeconfig context is declared with an
explicit (non-empty) namespace, then `mergedKubeConfig.Namespace()`
returns that *config file* namespace value, ignoring an explicit
`--namespace` command line flag.

See https://github.com/kubernetes/client-go/issues/288 for upstream
client-go bug/discussion.

This change works around the issue by just checking the Namespace
override ourselves before falling through to the regular function.

Further fixes #103