Fix RedisServer.Save(SaveType.BackgroundSave)

BGSAVE returns "Background saving started" upon success,
RedisServer.Save expected "OK".

StackExchange.Redis.Tests/Issues/BgSaveResponse.cs

+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using NUnit.Framework;
+
+namespace StackExchange.Redis.Tests.Issues
+{
+    [TestFixture]
+    public class BgSaveResponse : TestBase
+    {
+        [Test]
+        public void ShouldntThrowException()
+        {
+            using (var conn = Create(null, null, true))
+            {
+                var Server = GetServer(conn);
+                Server.Save(SaveType.BackgroundSave);
+            }
+        }
+    }
+}

StackExchange.Redis.Tests/StackExchange.Redis.Tests.csproj

@@ -71,6 +71,7 @@
     <Compile Include="Expiry.cs" />
     <Compile Include="Config.cs" />
     <Compile Include="FloatingPoint.cs" />
+    <Compile Include="Issues\BGSAVEResponse.cs" />
     <Compile Include="Issues\Issue6.cs" />
     <Compile Include="Issues\SO22786599.cs" />
     <Compile Include="Keys.cs" />

StackExchange.Redis/StackExchange/Redis/RedisLiterals.cs

@@ -65,6 +65,7 @@ public static readonly RedisValue
 
         public static readonly byte[] BytesOK = Encoding.UTF8.GetBytes("OK");
         public static readonly byte[] BytesPONG = Encoding.UTF8.GetBytes("PONG");
+        public static readonly byte[] BytesBackgroundSavingStarted = Encoding.UTF8.GetBytes("Background saving started");
         public static readonly byte[] ByteWildcard = { (byte)'*' };
         internal static RedisValue Get(Bitwise operation)
         {

StackExchange.Redis/StackExchange/Redis/RedisServer.cs

@@ -251,13 +251,13 @@ public void MakeMaster(ReplicationChangeOptions options, TextWriter log = null)
         public void Save(SaveType type, CommandFlags flags = CommandFlags.None)
         {
             var msg = GetSaveMessage(type, flags);
-            ExecuteSync(msg, ResultProcessor.DemandOK);
+            ExecuteSync(msg, GetSaveResultProcessor(type));
         }
 
         public Task SaveAsync(SaveType type, CommandFlags flags = CommandFlags.None)
         {
             var msg = GetSaveMessage(type, flags);
-            return ExecuteAsync(msg, ResultProcessor.DemandOK);
+            return ExecuteAsync(msg, GetSaveResultProcessor(type));
         }
 
         public bool ScriptExists(string script, CommandFlags flags = CommandFlags.None)
@@ -541,6 +541,17 @@ Message GetSaveMessage(SaveType type, CommandFlags flags = CommandFlags.None)
                 default:  throw new ArgumentOutOfRangeException("type");
             }
         }
+
+        ResultProcessor<bool> GetSaveResultProcessor(SaveType type)
+        {
+            switch (type)
+            {
+                case SaveType.BackgroundRewriteAppendOnlyFile: return ResultProcessor.DemandOK;
+                case SaveType.BackgroundSave: return ResultProcessor.BackgroundSaveStarted;
+                default: throw new ArgumentOutOfRangeException("type");
+            }
+        }
+
         struct KeysScanResult
         {
             public static readonly ResultProcessor<KeysScanResult> Processor = new KeysResultProcessor();

StackExchange.Redis/StackExchange/Redis/ResultProcessor.cs

@@ -13,12 +13,13 @@ abstract class ResultProcessor
         public static readonly ResultProcessor<bool>
             Boolean = new BooleanProcessor(),
             DemandOK = new ExpectBasicStringProcessor(RedisLiterals.BytesOK),
-            DemandPONG = new ExpectBasicStringProcessor("PONG"),
+            DemandPONG = new ExpectBasicStringProcessor(RedisLiterals.BytesPONG),
             DemandZeroOrOne = new DemandZeroOrOneProcessor(),
             AutoConfigure = new AutoConfigureProcessor(),
             TrackSubscriptions = new TrackSubscriptionsProcessor(),
             Tracer = new TracerProcessor(false),
-            EstablishConnection = new TracerProcessor(true);
+            EstablishConnection = new TracerProcessor(true),
+            BackgroundSaveStarted = new ExpectBasicStringProcessor(RedisLiterals.BytesBackgroundSavingStarted);
 
         public static readonly ResultProcessor<byte[]>
             ByteArray = new ByteArrayProcessor(),