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/common/server.rs

@@ -22,10 +22,10 @@ header! {
     /// use hyper::header::{Headers, Server};
     ///
     /// let mut headers = Headers::new();
-    /// headers.set(Server("hyper/0.5.2".to_owned()));
+    /// headers.set(Server::new("hyper/0.5.2"));
     /// ```
     // TODO: Maybe parse as defined in the spec?
-    (Server, "Server") => [String]
+    (Server, "Server") => Cow[str]
 
     test_server {
         // Testcase from RFC