PROTON-646: Fix when Perl sets a message body to be BOOL:False.

Had to work around how checking whether a body value was defined would
return false if it were defined but set to false.

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1617835 13f79535-47bb-0310-9956-ffa450edef68