feat(client): Response.status() now returns a StatusCode

Previously, it would return `&StatusCode`. Returning a reference was
actually bigger than the enum itself, and prevented using `Into` on the
return result directly.

BREAKING CHANGE: If you were explicitly checking the status, such as
  with an equality comparison, you will need to use the value instead of a
  reference.

src/client/request.rs

@@ -36,7 +36,7 @@ impl<B> Request<B> {
 
     /// Read the Request Version.
     #[inline]
-    pub fn version(&self) -> &HttpVersion { &self.version }
+    pub fn version(&self) -> HttpVersion { self.version }
 
     /// Read the Request headers.
     #[inline]