Return error in Readdir on regular mem file

Fixes #169

afero_test.go

@@ -570,6 +570,25 @@ func TestReaddir(t *testing.T) {
 	}
 }
 
+// https://github.com/spf13/afero/issues/169
+func TestReaddirRegularFile(t *testing.T) {
+	defer removeAllTestFiles(t)
+	for _, fs := range Fss {
+		f := tmpFile(fs)
+		defer f.Close()
+
+		_, err := f.Readdirnames(-1)
+		if err == nil {
+			t.Fatal("Expected error")
+		}
+
+		_, err = f.Readdir(-1)
+		if err == nil {
+			t.Fatal("Expected error")
+		}
+	}
+}
+
 type myFileInfo []os.FileInfo
 
 func (m myFileInfo) String() string {

mem/file.go

@@ -131,6 +131,9 @@ func (f *File) Sync() error {
 }
 
 func (f *File) Readdir(count int) (res []os.FileInfo, err error) {
+	if !f.fileData.dir {
+		return nil, &os.PathError{Op: "readdir", Path: f.fileData.name, Err: errors.New("not a dir")}
+	}
 	var outLength int64
 
 	f.fileData.Lock()