MemMapFs not checking for permissions

[staticdev]

How to reproduce:

package main

import (
    "log"

    "github.com/spf13/afero"
)

func main() {
    folderName := "secret"
    fsys := afero.NewMemMapFs()
    fsys.Mkdir(folderName, 0700)
    afero.WriteFile(fsys, "secret/filename", []byte(""), 0700)

    _, err := fsys.Stat("secret/filename")
    if err != nil {
        log.Fatal(err)
    }
    fsys.Chmod("secret", 0000)
    _, err = fsys.Stat("secret/filename")
    if err != nil {
        log.Fatal(err)
    }
}

If you do the same with NewOsFs() instead of NewMemMapFs() you get the error on second Stat (which is the intended behavior since the secret folder has no read permissions).

May be related to https://github.com/spf13/afero/issues/327