AMQP-364 Revert Long MesageProperties to long

API changes to support the message builder (with setIfAbsent) breaks
existing Apps. Use a boolean instead to determine if the header has been
explicitly set.

JIRA: https://jira.springsource.org/browse/AMQP-364

Cause: 22801e674f0e0c401d055a145381573308785790