Implementation of http.File and http.FileSystem doesn't handle directories correctly

[stoewer]

When opening a directory from a packr.Box using Open the resulting file object returns wrong information about this directory:

1. IsDir() returns false
2. ReadDir(-1) does not return os.FileInfo about files in this directory.

Assuming a folder structure like this:

resources
 |- testdata
     |- test_a.txt
     |- test_b.txt

The following small program illustrates this behavior:

func main() {
	httpDir := http.Dir("./resources")
	fmt.Println(listFiles(httpDir, "./testdata"))

	packrDir := packr.New("box", "./resources")
	fmt.Println(listFiles(packrDir, "./testdata"))
}

func listFiles(fs http.FileSystem, name string) []string {
	file, err := fs.Open(name)
	if err != nil {
		panic(err)
	}
	info, err := file.Stat()
	if err != nil {
		panic(err)
	}
	if !info.IsDir() {
		return []string{info.Name()}
	}
	childInfo, err := file.Readdir(-1)
	if err != nil {
		panic(err)
	}
	var names []string
	for _, info := range childInfo {
		if !info.IsDir() {
			names = append(names, info.Name())
		}
	}
	return names
}

The program prints out:

[test_a.txt test_b.txt]
[./testdata]

But of course the second line is expected to be identical to the first one.