Add test for #823

This is working as-design as ITransaction : IBatch and it's not safe in either, but adding a test to ensure it's performing correctly.

StackExchange.Redis.Tests/Transactions.cs

-using System.Threading.Tasks;
+using System;
+using System.Threading.Tasks;
 using Xunit;
 using Xunit.Abstractions;
 
@@ -25,6 +26,19 @@ public void BasicEmptyTran()
             }
         }
 
+        [Fact]
+        public void NestedTransactionThrows()
+        {
+            using (var muxer = Create())
+            {
+                var db = muxer.GetDatabase();
+                object asyncState = new object();
+                var tran = db.CreateTransaction();
+                var redisTransaction = Assert.IsType<RedisTransaction>(tran);
+                Assert.Throws<NotSupportedException>(() => redisTransaction.CreateTransaction(null));
+            }
+        }
+
         [Theory]
         [InlineData(false, false, true)]
         [InlineData(false, true, false)]
@@ -856,4 +870,4 @@ public void CombineFireAndForgetAndRegularAsyncInTransaction()
             }
         }
     }
-}
\ No newline at end of file
+}