Fine-tune default mime type in message broker config

Issue: SPR-11370
Add null check after message conversion

AbstractMessageSendingTemplate now checks if MessageConverter.toMessage
returns null and raises an exception.

Issue: SPR-11370
GenericConversionService matches converters by full generic target type, allowing for the registration of multiple converters from the same source type to different collection types

Issue: SPR-11369
Defensively handle ServletRequestAttributes casting in requestDestroyed callback

Issue: SPR-11378
Polishing

Issue: SPR-11369
Fix failing test