Make is_write_vectored return true for BufWriter

BufWriter provides an efficient implementation of
write_vectored also when the underlying writer does not
support vectored writes.

Author: mzabaluev

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

@@ -383,7 +383,7 @@ impl<W: Write> Write for BufWriter<W> {
     }
 
     fn is_write_vectored(&self) -> bool {
-        self.get_ref().is_write_vectored()
+        true
     }
 
     fn flush(&mut self) -> io::Result<()> {