WrapProccess on ClusterClient does not work if client has been copied

Test case:
```
client := redis.NewClusterClient(...)
client = client.WithContext(context.Background())
client.WrapProcess(...)
```

At this point the wrapper function will not be called. However, if we remove the `WithContext` call, it is called as expected.

The non-cluster client doesn't seem to have this issue.
