Cross origin (JSON based) XHR proxy requests are not forwarded properly.

The bodyParser middleware would parse the JSON properly, however the
`form` option in `request` was being used, and it was sent as a
'application/x-www-form-urlencoded' request. Instead it should be
stringified and set to the `request`'s `body` option.

This fixes another issue pointed out in:

https://github.com/blackberry/Ripple-UI/issues/693