Fix JavaMail Dependency

`ImapMessage` has a direct dependency on `ReadableMime`. It is
not clear why this started failing only on the MJATS41 build
but changing the `mailapi` dependency to `compile` fixes it
(tested by pointing the plan to my repo).
INT-3369: Syslog Converter `asMap` Option

JIRA: https://jira.spring.io/browse/INT-3369

INT-3369: add option asMap in DefaultMessageConverter
* add test

GH-1250