tweak transaction extension method

src/DotNetCore.CAP.MongoDB/ICapTransaction.MongoDB.cs

@@ -45,6 +45,11 @@ namespace DotNetCore.CAP
         public static ICapTransaction Begin(this ICapTransaction transaction,
             IClientSessionHandle dbTransaction, bool autoCommit = false)
         {
+            if (!dbTransaction.IsInTransaction)
+            {
+                dbTransaction.StartTransaction();
+            }
+           
             transaction.DbTransaction = dbTransaction;
             transaction.AutoCommit = autoCommit;