Update defaultUsage

This commit updates defaultUsage to match the current state in golang's flag.go.

Fixes spf13#242

flag.go

@@ -666,12 +666,17 @@ func PrintDefaults() {
 }
 
 // defaultUsage is the default function to print a usage message.
-func defaultUsage(f *FlagSet) {
+func (f *FlagSet) defaultUsage() {
+	if f.name == "" {
+		fmt.Fprintf(f.Output(), "Usage:\n")
+	} else {
+		fmt.Fprintf(f.Output(), "Usage of %s:\n", f.name)
+	}
 	fmt.Fprintf(f.Output(), "Usage of %s:\n", f.name)
 	f.PrintDefaults()
 }
 
-// NOTE: Usage is not just defaultUsage(CommandLine)
+// NOTE: Usage is not just CommandLine.defaultUsage()
 // because it serves (via godoc flag Usage) as the example
 // for how to write your own usage function.
 
@@ -680,7 +685,7 @@ func defaultUsage(f *FlagSet) {
 // By default it prints a simple header and calls PrintDefaults; for details about the
 // format of the output and how to control it, see the documentation for PrintDefaults.
 var Usage = func() {
-	fmt.Fprintf(os.Stderr, "Usage of %s:\n", os.Args[0])
+	fmt.Fprintf(CommandLine.Output(), "Usage of %s:\n", os.Args[0])
 	PrintDefaults()
 }
 
@@ -828,7 +833,7 @@ func (f *FlagSet) usage() {
 	if f == CommandLine {
 		Usage()
 	} else if f.Usage == nil {
-		defaultUsage(f)
+		f.defaultUsage()
 	} else {
 		f.Usage()
 	}