allow BufReader::peek to be called on unsized types

Author: lolbinarycat

library/std/src/io/buffered/bufreader.rs

@@ -117,7 +117,9 @@ impl<R: Read> BufReader<R> {
     /// assert_eq!(&s, "hello");
     /// ```
     #[unstable(feature = "bufreader_peek", issue = "128405")]
-    pub fn peek(&mut self, n: usize) -> io::Result<&[u8]> {
+    pub fn peek(&mut self, n: usize) -> io::Result<&[u8]>
+    where R: ?Sized,
+    {
         assert!(n <= self.capacity());
         while n > self.buf.buffer().len() {
             if self.buf.pos() > 0 {