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/client/mod.rs

@@ -40,7 +40,7 @@
 //! println!("status: {}", res.status());
 //!
 //! // Concatenate the body stream into a single buffer...
-//! let buf = hyper::body::to_bytes(res.into_body()).await?;
+//! let buf = hyper::body::to_bytes(res).await?;
 //!
 //! println!("body: {:?}", buf);
 //! # Ok(())