fix kubectl cp command error.

it happens when copy directory to pod and the directory
path ends with '/'.
for example:
kubectl cp /XX/XX/ XX-POD:/XX/XX

modified:  pkg/kubectl/cmd/cp.go
modified:  pkg/kubectl/cmd/cp_test.go

pkg/kubectl/cmd/cp.go

@@ -195,6 +195,8 @@ func makeTar(filepath string, writer io.Writer) error {
 	// TODO: use compression here?
 	tarWriter := tar.NewWriter(writer)
 	defer tarWriter.Close()
+
+	filepath = path.Clean(filepath)
 	return recursiveTar(path.Dir(filepath), path.Base(filepath), tarWriter)
 }