QPID-4565: Setting message content turned maps and lists into strings.

When setting content for a message, the Ruby code attempted to convert
anything that was a symbol to a string. But it was overzealous and
converted everything to a string, including map values that were
themselves maps or lists.

This fix now only converts symbols to strings. It steps down through
arrays or lists and only converts its elements that are symbols to
strings, leaving anything else as is.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1442329 13f79535-47bb-0310-9956-ffa450edef68