`Request.application` could reuse existing request

When a `Request` is created, it sets `environ["werkzeug.request"] = self` if `populate_request=True` (the default). The `Request.application` wrapper can check for this and use it instead of creating a new object. This would allow middleware to be built with Werkzeug that _can_ read `data` or `form` and have that (and cached parsed headers) still available to the application.