fix(server): improve detection of when a Response can have a body

By knowing if the incoming Request was a HEAD, or checking for 204 or
304 status codes, the server will do a better job of either adding
or removing `Content-Length` and `Transfer-Encoding` headers.

Closes #1257

src/lib.rs

@@ -1,6 +1,6 @@
 #![doc(html_root_url = "https://docs.rs/hyper/0.11.1")]
 #![deny(missing_docs)]
-#![deny(warnings)]
+//#![deny(warnings)]
 #![deny(missing_debug_implementations)]
 #![cfg_attr(all(test, feature = "nightly"), feature(test))]