[bug] WinCred kr.Keys() return empty when kr.Set() doesn't call before

I have set `keyring:default:key1`, `keyring:default:key2` already, and can be found in windows Credential Manager.
then test following code:

```
kr, err := keyring.Open(keyring.Config{
AllowedBackends: []keyring.BackendType{keyring.WinCredBackend},
})
if err != nil {
t.Fatal(err)
}
keys, err := kr.Keys()
if err != nil {
t.Fatal(err)
}
```

it would return empty keys. but if I call kr.Set() before kr.Keys(), it would return `keyring:default:key1` and `keyring:default:key2`

```
kr, err := keyring.Open(keyring.Config{
AllowedBackends: []keyring.BackendType{keyring.WinCredBackend},
})
if err != nil {
t.Fatal(err)
}
item1 := keyring.Item{
Key: "test3",
Data: []byte("loose lips sink ships"),
}
err = kr.Set(item1)
if err != nil {
t.Fatal(err)
}
keys, err := kr.Keys()
if err != nil {
t.Fatal(err)
}
```