io: Forward poll_write_buf on BufReader (#2654)

For some yet unknown reason using the default on a wrapped `Bufreader<TcpStream>`
causes the hyper server to sometimes fail to send the entire body in the
response.

This fixes that problem for us and ensures that hyper has a chance to
use vectored IO (making it a good change regardless of the mentioned
bug)

tokio/src/io/util/buf_reader.rs

@@ -1,6 +1,7 @@
 use crate::io::util::DEFAULT_BUF_SIZE;
 use crate::io::{AsyncBufRead, AsyncRead, AsyncWrite};
 
+use bytes::Buf;
 use pin_project_lite::pin_project;
 use std::io::{self, Read};
 use std::mem::MaybeUninit;
@@ -162,6 +163,14 @@ impl<R: AsyncRead + AsyncWrite> AsyncWrite for BufReader<R> {
         self.get_pin_mut().poll_write(cx, buf)
     }
 
+    fn poll_write_buf<B: Buf>(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut B,
+    ) -> Poll<io::Result<usize>> {
+        self.get_pin_mut().poll_write_buf(cx, buf)
+    }
+
     fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
         self.get_pin_mut().poll_flush(cx)
     }