Fix kubectl get error when both all-namespaces and namespace are provided.

As per kubectl get help, --namespace should be ignored with all-namespaces,
but kubectl get pods --all-namespaces --namespace=<name-space> gives
following error:

"the namespace from the provided object "default" does not match the
namespace "". You must pass '--namespace=default' to perform this
operation."

This commit fixes this error issue.

hack/test-cmd.sh

@@ -1491,6 +1491,33 @@ __EOF__
   # Command
   kubectl get pods --sort-by="{metadata.name}"
 
+  ############################
+  # Kubectl --all-namespaces #
+  ############################
+
+  # Pre-condition: the "default" namespace exists
+  kube::test::get_object_assert namespaces "{{range.items}}{{$id_field}}:{{end}}" 'default:'
+
+  ### Create POD
+  # Pre-condition: no POD exists
+  kube::test::get_object_assert pods "{{range.items}}{{$id_field}}:{{end}}" ''
+  # Command
+  kubectl create "${kube_flags[@]}" -f docs/admin/limitrange/valid-pod.yaml
+  # Post-condition: valid-pod is created
+  kube::test::get_object_assert pods "{{range.items}}{{$id_field}}:{{end}}" 'valid-pod:'
+
+  ### Verify a specific namespace is ignored when all-namespaces is provided
+  # Command
+  kubectl get pods --all-namespaces --namespace=default
+
+  ### Clean up
+  # Pre-condition: valid-pod exists
+  kube::test::get_object_assert pods "{{range.items}}{{$id_field}}:{{end}}" 'valid-pod:'
+  # Command
+  kubectl delete "${kube_flags[@]}" pod valid-pod --grace-period=0
+  # Post-condition: valid-pod doesn't exist
+  kube::test::get_object_assert pods "{{range.items}}{{$id_field}}:{{end}}" ''
+
   kube::test::clear_all
 }