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/location.rs

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