feat(server): add Response.send to write a sized body

Closes #446

examples/hello.rs

@@ -2,15 +2,12 @@
 extern crate hyper;
 extern crate env_logger;
 
-use std::io::Write;
 use hyper::server::{Request, Response};
 
 static PHRASE: &'static [u8] = b"Hello World!";
 
 fn hello(_: Request, res: Response) {
-    let mut res = res.start().unwrap();
-    res.write_all(PHRASE).unwrap();
-    res.end().unwrap();
+    res.send(PHRASE).unwrap();
 }
 
 fn main() {