[#302] pkg/reputation: Document default values set in NewPeerID

Document field values of instance constructed via `NewPeerID`.
Assert the values in corresponding unit test.

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

pkg/reputation/peer.go

@@ -12,6 +12,9 @@ import (
 type PeerID reputation.PeerID
 
 // NewPeerID creates and returns blank PeerID.
+//
+// Defaults:
+//  - publicKey: nil.
 func NewPeerID() *PeerID {
 	return PeerIDFromV2(new(reputation.PeerID))
 }