Add SecretUpdate method to client

closes #28678

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)

Update cli/command/service/update_test.go

Fixes test build error:

  secretAPIClientMock does not implement "github.com/docker/docker/client".SecretAPIClient (missing SecretUpdate method)

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)

client/secret_update_test.go

@@ -0,0 +1,49 @@
+package client
+
+import (
+	"bytes"
+	"fmt"
+	"io/ioutil"
+	"net/http"
+	"strings"
+	"testing"
+
+	"golang.org/x/net/context"
+
+	"github.com/docker/docker/api/types/swarm"
+)
+
+func TestSecretUpdateError(t *testing.T) {
+	client := &Client{
+		client: newMockClient(errorMock(http.StatusInternalServerError, "Server error")),
+	}
+
+	err := client.SecretUpdate(context.Background(), "secret_id", swarm.Version{}, swarm.SecretSpec{})
+	if err == nil || err.Error() != "Error response from daemon: Server error" {
+		t.Fatalf("expected a Server Error, got %v", err)
+	}
+}
+
+func TestSecretUpdate(t *testing.T) {
+	expectedURL := "/secrets/secret_id/update"
+
+	client := &Client{
+		client: newMockClient(func(req *http.Request) (*http.Response, error) {
+			if !strings.HasPrefix(req.URL.Path, expectedURL) {
+				return nil, fmt.Errorf("Expected URL '%s', got '%s'", expectedURL, req.URL)
+			}
+			if req.Method != "POST" {
+				return nil, fmt.Errorf("expected POST method, got %s", req.Method)
+			}
+			return &http.Response{
+				StatusCode: http.StatusOK,
+				Body:       ioutil.NopCloser(bytes.NewReader([]byte("body"))),
+			}, nil
+		}),
+	}
+
+	err := client.SecretUpdate(context.Background(), "secret_id", swarm.Version{}, swarm.SecretSpec{})
+	if err != nil {
+		t.Fatal(err)
+	}
+}