Fix middleware spec for recent versions of Rack

It's not valid to pass a non-string as part of the response body.

Signed-off-by: Evan Owen <kainosnoema@gmail.com>
Remove XML support, multi_xml gem

We should focus on JSON support and not require every user to depend
on the multi_xml gem unless they need it. Adding the gem and a simple
XML parser middleware is trivial.

Signed-off-by: Evan Owen <kainosnoema@gmail.com>
Merge pull request #34 from stephencelis/remove-xml

Remove XML support, multi_xml gem