`body` data not passed correctly to the express app

Currently, when using express, the `req.body` data is incorrectly passed to the express application. I am getting the gibberish trash data from somewhere passed into the request. This problem does not exists if I use `getCurrentInvoke` to get the `body` data, which will be correct.

Propose to fix this by changing this line here https://github.com/vendia/serverless-express/blob/mainline/src/frameworks/express.js#L2 from `app.handle` to `app`, which should solve the issue. See https://github.com/expressjs/express/blob/master/lib/express.js#L38.

Currently I am using express v4, not sure if this will be the case for express v5 or even prior versions.

I will be more than happy to work on this issue (and other issues as a contributor).

Maybe we can make the `event` & `context` data not available for the express `Request` object in the future as well for convenience.