feat(client): Response.status() now returns a StatusCode

Previously, it would return `&StatusCode`. Returning a reference was
actually bigger than the enum itself, and prevented using `Into` on the
return result directly.

BREAKING CHANGE: If you were explicitly checking the status, such as
  with an equality comparison, you will need to use the value instead of a
  reference.

tests/client.rs

@@ -89,7 +89,7 @@ macro_rules! test {
             let work = res.join(rx).map(|r| r.0);
 
             let res = core.run(work).unwrap();
-            assert_eq!(res.status(), &StatusCode::$client_status, "status is invalid");
+            assert_eq!(res.status(), StatusCode::$client_status, "status is invalid");
             $(
                 assert_eq!(res.headers().get(), Some(&$response_headers), "headers are invalid");
             )*