`AbstractMessageSendingTemplate` ignores headers in `convertAndSend()` variant

## Overview

While working on #36118, I noticed that the 
`convertAndSend(Object, Map<String, Object>, MessagePostProcessor)` variant in `AbstractMessageSendingTemplate` ignores the supplied headers map.

Consequently, concrete implementations like `GenericMessagingTemplate` which do not override that method will effectively drop the headers.

## Related Issues

- #32501
- #36118
