feat(headers): adds re-parsing ability when getting typed headers

BREAKING CHANGE: added requirement that all HeaderFormat implementations
  must also be fmt::Debug. This likely as easy as slapping
  #[derive(Debug)] on to any custom headers.

benches/client.rs

@@ -50,7 +50,7 @@ impl Write for MockStream {
     }
 }
 
-#[derive(Clone)]
+#[derive(Clone, Debug)]
 struct Foo;
 
 impl hyper::header::Header for Foo {