force message buffers to strings before decoding

Although the top comment in anyjson suggests otherwise, not all json decoders can deal with kombu's message body buffers. This PR introduces a fix outlined at the bottom of http://kombu.readthedocs.org/en/latest/userguide/serialization.html

For reproducing the error, install libjson2 and watch the decoding errors, preferably by means of #281