Return JSON to naive HTTP clients like browsers or curl.

Looks like this broke after https://github.com/berkshelf/berkshelf-api/commit/5fec073f5131bde75ad44128001cbfcdc3c62baa switch from `format` to `default_format`, which checks the Accept header to determine what format to use. Thanks to XHTML, most browsers and similar clients include `application/xml` in their types list. Not sure if this is too heavy a hammer, but it does seem to work.