feat(headers): header ergonomics

- Reexports all common::* as header::*
- Most headers implement Deref where it makes sense.

Closes #156

src/server/response.rs

@@ -83,7 +83,7 @@ impl<'a> Response<'a, Fresh> {
         match self.headers.get::<common::ContentLength>() {
             Some(cl) => {
                 chunked = false;
-                len = cl.len();
+                len = **cl;
             },
             None => ()
         };