Use correct exception type

Author: SeanFeldman

src/Microsoft.Azure.ServiceBus/Core/MessageBatch.cs

@@ -134,7 +134,7 @@ private void ThrowIfDisposed()
         {
             if (result == null)
             {
-                throw new Exception("MessageBatch is has been disposed and cannot be re-used.");
+                throw new ObjectDisposedException("MessageBatch is has been disposed and cannot be re-used.");
             }
         }