refactor(header): change name of deref! to __hyper__deref!

The header is largely for internal use, however, it must be exported
because the header! macro uses it. As deref! is also a particularly
general name, this renames it to __hyper__deref! to prevent name
clashes.

BREAKING CHANGE: If you use deref! from the header module, you'll need
  to switch to using __hyper__deref!.

src/header/common/set_cookie.rs

@@ -57,7 +57,7 @@ use cookie::CookieJar;
 #[derive(Clone, PartialEq, Debug)]
 pub struct SetCookie(pub Vec<Cookie>);
 
-deref!(SetCookie => Vec<Cookie>);
+__hyper__deref!(SetCookie => Vec<Cookie>);
 
 impl Header for SetCookie {
     fn header_name() -> &'static str {