QPID-5481: Messaging API Update - 1513536 Message constructor and content getters added

The .NET Binding already had the bytes constructor and the getContentBytes methods.
This patch adds getContentObject/setContentObject interfaces.


git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1558619 13f79535-47bb-0310-9956-ffa450edef68