Set writer when running command as app

Also add test from https://github.com/codegangsta/cli/pull/202 with
slight modifications.

app_test.go

@@ -1,10 +1,12 @@
 package cli_test
 
 import (
+	"bytes"
 	"flag"
 	"fmt"
 	"io"
 	"os"
+	"strings"
 	"testing"
 
 	"github.com/codegangsta/cli"
@@ -621,3 +623,57 @@ func TestGlobalFlagsInSubcommands(t *testing.T) {
 
 	expect(t, subcommandRun, true)
 }
+
+func TestApp_Run_CommandWithSubcommandHasHelpTopic(t *testing.T) {
+	var subcommandHelpTopics = [][]string{
+		{"command", "foo", "--help"},
+		{"command", "foo", "-h"},
+		{"command", "foo", "help"},
+	}
+
+	for _, flagSet := range subcommandHelpTopics {
+		t.Logf("==> checking with flags %v", flagSet)
+
+		app := cli.NewApp()
+		buf := new(bytes.Buffer)
+		app.Writer = buf
+
+		subCmdBar := cli.Command{
+			Name:  "bar",
+			Usage: "does bar things",
+		}
+		subCmdBaz := cli.Command{
+			Name:  "baz",
+			Usage: "does baz things",
+		}
+		cmd := cli.Command{
+			Name:        "foo",
+			Description: "descriptive wall of text about how it does foo things",
+			Subcommands: []cli.Command{subCmdBar, subCmdBaz},
+		}
+
+		app.Commands = []cli.Command{cmd}
+		err := app.Run(flagSet)
+
+		if err != nil {
+			t.Error(err)
+		}
+
+		output := buf.String()
+		t.Logf("output: %q\n", buf.Bytes())
+
+		if strings.Contains(output, "No help topic for") {
+			t.Errorf("expect a help topic, got none: \n%q", output)
+		}
+
+		for _, shouldContain := range []string{
+			cmd.Name, cmd.Description,
+			subCmdBar.Name, subCmdBar.Usage,
+			subCmdBaz.Name, subCmdBaz.Usage,
+		} {
+			if !strings.Contains(output, shouldContain) {
+				t.Errorf("want help to contain %q, did not: \n%q", shouldContain, output)
+			}
+		}
+	}
+}