[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)
}
```

Look at the WinCred backend implementation in the keyring library, likely in a file like wincred.go. The bug is that kr.Keys() returns empty unless kr.Set() is called first. Start by examining how the Windows Credential Manager is queried for existing keys and how the keyring instance caches or initializes its state. Run the provided test code on a Windows system to reproduce the issue, then trace through the Keys() method to see where it fails to retrieve pre-existing credentials.

Scritto dal modello di indicizzazione a partire dal testo della issue.