Report an error if no flag(s) set in service update

For now if no flag(s) set, service update will still try to
do an update, it should return an error instead.

[issue 286](#286)

pkg/kn/commands/service/service_update.go

@@ -29,7 +29,7 @@ func NewServiceUpdateCommand(p *commands.KnParams) *cobra.Command {
 	var waitFlags commands.WaitFlags
 
 	serviceUpdateCommand := &cobra.Command{
-		Use:   "update NAME",
+		Use:   "update NAME [flags]",
 		Short: "Update a service.",
 		Example: `
   # Updates a service 'mysvc' with new environment variables
@@ -50,6 +50,10 @@ func NewServiceUpdateCommand(p *commands.KnParams) *cobra.Command {
 				return err
 			}
 
+			if cmd.Flags().NFlag() == 0 {
+				return fmt.Errorf("Flag(s) not set\nUsage: %s", cmd.Use)
+			}
+
 			client, err := p.NewClient(namespace)
 			if err != nil {
 				return err

pkg/kn/commands/service/service_update_test.go

@@ -88,6 +88,26 @@ func fakeServiceUpdate(original *v1alpha1.Service, args []string, sync bool) (
 	return
 }
 
+func TestServcieUpdateNoFlags(t *testing.T) {
+	orig := newEmptyService()
+
+	action, _, _, err := fakeServiceUpdate(orig, []string{
+		"service", "update", "foo"}, false)
+
+	if action != nil {
+		t.Errorf("Unexpected action if no flag(s) set")
+	}
+
+	if err != nil {
+		expectedErrMsg := "Flag(s) not set"
+		if !strings.Contains(err.Error(), expectedErrMsg) {
+			t.Errorf("Missing %s in %s", expectedErrMsg, err.Error())
+		}
+	} else {
+		t.Errorf("Error is expected if no flag(s) set")
+	}
+}
+
 func TestServiceUpdateImageSync(t *testing.T) {
 	orig := newEmptyService()