Allow text/javascript response.

Simply because the Dropbox API returns text/javascript instead of application/json:

headers       => {
                   "cache-control"           => [["no-cache"]],
                   "connection"              => [["keep-alive"]],
                   "content-length"          => [[127]],
                   "content-type"            => [["text/javascript"]],
                   "date"                    => [["Mon, 05 May 2014 13:07:31 GMT"]],
                   "pragma"                  => [["no-cache"]],
                   "server"                  => [["nginx"]],
                   "x-dropbox-http-protocol" => [["None"]],
                   "x-dropbox-request-id"    => [["2063b64ae56844ebaee02289f267b393"]],
                   "x-frame-options"         => [["SAMEORIGIN"]],
                   "x-requestid"             => [["0b0c84030cc157d659d4244e2d668fd4"]],
                   "x-server-response-time"  => [[119]],
                 },