fix(via): prefer mapping request

[zacharygolba]

Prefers mapping request rather than working with the component parts individually.

I would like version 2 to ship with a bultin proxy middleware with nginx like arguments to a via::middleware::upstream function. Perhaps it will require a feature flag if changes to the serve function are required.

Keeping the request intact and introducing a Request::into_inner function is the first step in the only forseen feature work before writing docs, calling it version 2, and switching a more professional--predictable release cycle.

✌️