timer: impl Into<std::io::Error> for Elpased (#1321)

That convert Elpased to ErrorKind::TimedOut

tokio-timer/src/timeout.rs

@@ -220,3 +220,9 @@ impl fmt::Display for Elapsed {
 }
 
 impl std::error::Error for Elapsed {}
+
+impl Into<std::io::Error> for Elapsed {
+    fn into(self) -> std::io::Error {
+        std::io::ErrorKind::TimedOut.into()
+    }
+}