perf(http): reduce a copy when not using Chunked encoding

src/http/io.rs

@@ -242,9 +242,13 @@ impl<T: Write + ::vecio::Writev> AtomicWrite for T {
 */
 impl<T: Write> AtomicWrite for T {
     fn write_atomic(&mut self, bufs: &[&[u8]]) -> io::Result<usize> {
+        if bufs.len() == 1 {
+            self.write(bufs[0])
+        } else {
             let vec = bufs.concat();
             self.write(&vec)
         }
+    }
 }
 //}