feat(body): implement HttpBody for Request and Response

When the body type of a `Request` or `Response` implements `HttpBody`,
the `Request` or `Response` itself now implements `HttpBody`.

This allows writing things like `hyper::body::aggregate(req)` instead of
`hyper::body::aggregate(req.into_body())`.

Closes #2067

src/body/body.rs

@@ -18,7 +18,7 @@ type BodySender = mpsc::Sender<Result<Bytes, crate::Error>>;
 
 /// A stream of `Bytes`, used when receiving bodies.
 ///
-/// A good default [`HttpBody`](crates::body::HttpBody) to use in many
+/// A good default [`HttpBody`](crate::body::HttpBody) to use in many
 /// applications.
 #[must_use = "streams do nothing unless polled"]
 pub struct Body {