Fix tradeoffer error ( SteamLoginSecure)

Based on the 404 from @scholtzm

I added minor fixes so that this could be compatible with tradeoffer branch.

This pull request might have some small mistakes since this is my first one

Other than that this should work good!
should be:
```
tradeOfferManager = new TradeOfferManager(apiKey, sessionId, token); 
SubscribeTradeOffer(tradeOfferManager);
```