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
Use com.sun.mail:javax.mail for ReadableMime dependency

com.sun.mail:javax.mail is already part of the Spring IO Platform,
whereas com.sun.mail:mailapi is not.