Use dynamic dispatch for client Request and Response through Box<NetworkStream>

Also adds a convenience `abstract` method to NetworkStream for creating
Box<NetworkStream + Send> from a NetworkStream.

src/server/response.rs

@@ -66,7 +66,7 @@ impl Response<Fresh> {
             status: status::Ok,
             version: version::Http11,
             headers: header::Headers::new(),
-            body: BufferedWriter::new(box stream as Box<NetworkStream + Send>)
+            body: BufferedWriter::new(stream.abstract())
         }
     }