[#302] pkg/session: Document default values set in NewToken

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

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

pkg/session/session.go

@@ -24,6 +24,15 @@ func NewTokenFromV2(tV2 *session.SessionToken) *Token {
 }
 
 // NewToken creates and returns blank Token.
+//
+// Defaults:
+//  - body: nil;
+//  - id: nil;
+//  - ownerId: nil;
+//  - sessionKey: nil;
+//  - exp: 0;
+//  - iat: 0;
+//  - nbf: 0;
 func NewToken() *Token {
 	return NewTokenFromV2(new(session.SessionToken))
 }