Fix ERR only (P)SUBSCRIBE / (P)UNSUBSCRIBE / PING / QUIT allowed in t…

[Kilowhisky]

Fixes issue #694

I've confirmed that StackExchange.Redis isn't squawking about the error anymore and the pub/sub open/close doesn't happen on the server anymore either.

[Kilowhisky]

Hold on this PR actually, i found a way for it to start spitting out

System.InvalidOperationException: unexpected tracer reply to PING: BulkString: 26 bytes\r\n

Which tells me something isn't quite right with the implementation.

[Kilowhisky]

Yea, guess i need to figure out how to return a bulkArray, please stand by....

 case RedisCommand.PING:
                        // there are two different PINGs; "interactive" is a +PONG or +{your message},
                        // but subscriber returns a bulk-array of [ "pong", {your message} ]
                        switch (result.Type)
                        {
                            case ResultType.SimpleString:
                                happy = result.IsEqual(CommonReplies.PONG);
                                break;
                            case ResultType.MultiBulk:
                                if (result.ItemsCount == 2)
                                {
                                    var items = result.GetItems();
                                    happy = items[0].IsEqual(CommonReplies.PONG) && items[1].Payload.IsEmpty;
                                }
                                else
                                {
                                    happy = false;
                                }
                                break;
                            default:
                                happy = false;
                                break;
                        }
                        break;

[Kilowhisky]

Ready to merge