Fix off-by-one error in promote_to_ready (#41)

The `..` syntax creates a _half-open_ range (see https://doc.rust-lang.org/std/ops/struct.Range.html), so all that messing about with `n-1` in #39 and #40 was never actually necessary. This actually fixes the Conduit test I mentioned in #39 (comment); it no longer hangs.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

tower-balance/src/lib.rs

@@ -156,25 +156,27 @@ where
         debug!("promoting to ready: {}", n);
         // Iterate through the not-ready endpoints from right to left to prevent removals
         // from reordering services in a way that could prevent a service from being polled.
-        for idx in (0..n-1).rev() {
+        for idx in (0..n).rev() {
             let is_ready = {
                 let (_, svc) = self.not_ready
                     .get_index_mut(idx)
                     .expect("invalid not_ready index");;
                 svc.poll_ready().map_err(Error::Inner)?.is_ready()
             };
-
+            trace!("not_ready[{:?}]: is_ready={:?};", idx, is_ready);
             if is_ready {
-                debug!("promoting to ready");
+                debug!("not_ready[{:?}]: promoting to ready", idx);
                 let (key, svc) = self.not_ready
                     .swap_remove_index(idx)
                     .expect("invalid not_ready index");
                 self.ready.insert(key, svc);
             } else {
-                debug!("not promoting to ready");
+                debug!("not_ready[{:?}]: not promoting to ready", idx);
             }
         }
 
+        debug!("promoting to ready: done");
+
         Ok(())
     }