req.body should not be use it for form post

[jknack]

Form post as urlencoded or multipart must be parsed/read using .params or new .form alias from #496

{
  post("/", req -> {
    MyForm form = req.form(MyForm.class);
  });
}

While req.body must be used it for content types like json, xml, ..., etc.

{
  post("/", req -> {
    MyJsonObject object = req.body(MyJsonObject.class);
  });
}

Please note any existing app that use req.body for form post MUST BE UPDATED and use req.params() or req.form()

This is required to fix #444