PROTON-215: Fix python test error introduced by previous commits.

In interop.py, after decoding a message, Message.body is of <type 'str'> under
plain Python or proton-j Jython, but under proton-jni Jython it is a
<type 'org.apache.qpid.proton.amqp.Binary'>

Check the type and extract the Binary's data as a str if necessary.

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1451329 13f79535-47bb-0310-9956-ffa450edef68