Add send method to Response<Fresh>

[seanmonstar]

This method would be a guaranteed single write, so we could set the Content-Length header, write the body, and then end() the request.

Basically:

impl Response<Fresh> {
    fn send(self, buf: &[u8]) -> io::Result<usize> {
        self.headers.set(ContentLength(buf.len()));
        try!(self.body.write_all(buf));
        self.end();
        Ok(buf.len())
    }
}