Tests: add KeyIdleTime, and fix the null case

The return differs on OBJECT IDLETIME, it's a bulkstring null when the key does not exist...handle it accordingly.

src/StackExchange.Redis/Interfaces/IDatabase.cs

@@ -512,7 +512,7 @@ public interface IDatabase : IRedis, IDatabaseAsync
         /// <summary>
         /// Returns the time since the object stored at the specified key is idle (not requested by read or write operations)
         /// </summary>
-        /// <param name="key">The key to get the type of.</param>
+        /// <param name="key">The key to get the time of.</param>
         /// <param name="flags">The flags to use for this operation.</param>
         /// <returns>The time since the object stored at the specified key is idle</returns>
         /// <remarks>https://redis.io/commands/object</remarks>

src/StackExchange.Redis/Interfaces/IDatabaseAsync.cs

@@ -476,7 +476,7 @@ public interface IDatabaseAsync : IRedisAsync
         /// <summary>
         /// Returns the time since the object stored at the specified key is idle (not requested by read or write operations)
         /// </summary>
-        /// <param name="key">The key to get the type of.</param>
+        /// <param name="key">The key to get the time of.</param>
         /// <param name="flags">The flags to use for this operation.</param>
         /// <returns>The time since the object stored at the specified key is idle</returns>
         /// <remarks>https://redis.io/commands/object</remarks>

src/StackExchange.Redis/ResultProcessor.cs

@@ -295,6 +295,10 @@ public bool TryParse(RawResult result, out TimeSpan? expiry)
                             return true;
                         }
                         break;
+                    // e.g. OBJECT IDLETIME on a key that doesn't exist
+                    case ResultType.BulkString when result.IsNull:
+                        expiry = null;
+                        return true;
                 }
                 expiry = null;
                 return false;

tests/StackExchange.Redis.Tests/Keys.cs

-using System.Linq;
+using System;
+using System.Linq;
 using System.Text;
+using System.Threading.Tasks;
 using Xunit;
 using Xunit.Abstractions;
 
@@ -105,5 +107,28 @@ public void PrependAppend()
                 Assert.Equal("helloworld", (string)key3);
             }
         }
+
+        [Fact]
+        public async Task IdleTime()
+        {
+            using (var muxer = Create())
+            {
+                RedisKey key = Me();
+                var db = muxer.GetDatabase();
+                db.KeyDelete(key, CommandFlags.FireAndForget);
+                db.StringSet(key, "new value", flags: CommandFlags.FireAndForget);
+                await Task.Delay(2000).ForAwait();
+                var idleTime = db.KeyIdleTime(key);
+                Assert.True(idleTime > TimeSpan.Zero);
+
+                db.StringSet(key, "new value2", flags: CommandFlags.FireAndForget);
+                var idleTime2 = db.KeyIdleTime(key);
+                Assert.True(idleTime2 < idleTime);
+
+                db.KeyDelete(key);
+                var idleTime3 = db.KeyIdleTime(key);
+                Assert.Null(idleTime3);
+            }
+        }
     }
 }