refactor(header): make some headers more allocator friendly

Change the internal implementation of some simple headers to make them
more allocator friendly. Also add a constructor method to allow changing
the implementation in the future again.

The headers are:

- Location
- Referrer
- Server
- UserAgent

This change was suggested in [#1104].

BREAKING CHANGES:
- Old code that creates the header structs directly will stop working.
- It's not possible to implement DerefMut for a Cow<'static,str>. Code
that needs to modify header after creation will stop working.

src/header/mod.rs

@@ -148,7 +148,7 @@ fn test_get_type() {
     use ::header::{ContentLength, UserAgent};
 
     let len = ContentLength(5);
-    let agent = UserAgent("hyper".to_owned());
+    let agent = UserAgent::new("hyper");
 
     assert_eq!(TypeId::of::<ContentLength>(), len.get_type());
     assert_eq!(TypeId::of::<UserAgent>(), agent.get_type());