Add work-around for responses that are already decoded.

When the response type is ajax, rather than plain text, the response is
already json decoded. I'm guessing this happens somewhere in the li3
response processing, but I don't know where that behavior could be
easily changed yet. This is a simple work-around for those cases.
Merge pull request #45 from mdx-dev/li3_decoded_check

Add work-around for responses that are already decoded.