feat(headers): header ergonomics

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

Closes #156

src/header/mod.rs

@@ -21,6 +21,8 @@ use uany::{UncheckedAnyDowncast, UncheckedAnyMutDowncast};
 use http::{mod, LineEnding};
 use {HttpResult};
 
+pub use self::common::*;
+
 /// Common Headers
 pub mod common;