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/referer.rs

@@ -21,16 +21,16 @@ header! {
     /// use hyper::header::{Headers, Referer};
     ///
     /// let mut headers = Headers::new();
-    /// headers.set(Referer("/People.html#tim".to_owned()));
+    /// headers.set(Referer::new("/People.html#tim"));
     /// ```
     /// ```
     /// use hyper::header::{Headers, Referer};
     ///
     /// let mut headers = Headers::new();
-    /// headers.set(Referer("http://www.example.com/index.html".to_owned()));
+    /// headers.set(Referer::new("http://www.example.com/index.html"));
     /// ```
     // TODO Use URL
-    (Referer, "Referer") => [String]
+    (Referer, "Referer") => Cow[str]
 
     test_referer {
         // Testcase from the RFC