Fixed: dispose SqlBulkBatchWriter in sink

A Dispose() call for _sqlBulkBatchWriter was missing in MSSqlServerSink class.

Author: ckadluba

src/Serilog.Sinks.MSSqlServer/Sinks/MSSqlServer/MSSqlServerSink.cs

@@ -151,6 +151,7 @@ protected virtual void Dispose(bool disposing)
             {
                 if (disposing)
                 {
+                    _sqlBulkBatchWriter.Dispose();
                     _sqlLogEventWriter.Dispose();
                 }