Return nil in CopyOnWriteFs.MkdirAll when directory exists

Fixes #191

copyOnWriteFs.go

@@ -282,7 +282,8 @@ func (u *CopyOnWriteFs) MkdirAll(name string, perm os.FileMode) error {
 		return u.layer.MkdirAll(name, perm)
 	}
 	if dir {
-		return ErrFileExists
+		// This is in line with how os.MkdirAll behaves.
+		return nil
 	}
 	return u.layer.MkdirAll(name, perm)
 }

copyOnWriteFs_test.go

@@ -36,13 +36,12 @@ func TestCopyOnWrite(t *testing.T) {
 			t.Errorf("Mkdir: Got %q for %T", err, fs)
 		}
 
-		// https://github.com/spf13/afero/issues/191
-		if _, ok := fs.(*OsFs); !ok {
+		// MkdirAll does not return an error when the directory already exists
 		err = fs.MkdirAll(dir, 0744)
-			if err == nil || !os.IsExist(err) {
+		if err != nil {
 			t.Errorf("MkdirAll:  Got %q for %T", err, fs)
 		}
-		}
+
 	}
 }