Fix RedisServer.Save(SaveType.BackgroundSave)

Hi Marc,

First, thanks for the good work !

I stumbled upon what seems to be a small protocol bug when calling BGSAVE.
The library expects the usual "OK" response from the server, just like BGREWRITEAOF, but in the case of this call, Redis will return "Background saving started" as success.

Occurred on version 1.0.247.0 (fresh from NuGet), Redis 2.6.12.
The reproduction code is simple enough :

``` c#
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using StackExchange.Redis;

namespace RedisBGSAVE
{
    class Program
    {
        static void Main(string[] args)
        {
            var Options = new ConfigurationOptions();
            Options.EndPoints.Add("localhost", 6379);
            Options.AllowAdmin = true;
            var Multiplexer = ConnectionMultiplexer.Connect(Options);
            var Server = Multiplexer.GetServer(Multiplexer.GetEndPoints().First());
            Server.Save(SaveType.BackgroundSave);
        }
    }
}
```

This will throw a
```
[StackExchange.Redis.RedisConnectionException]:
    {"Unexpected response to BGSAVE: SimpleString: Background saving started"}
```