Async topic is passed to vows with topic-less subcontext

Fixes #53

lib/vows/suite.js

@@ -163,7 +163,13 @@ this.Suite.prototype = new(function () {
                               };
                           }(topic));
                       }
-                      topic = ctx.emitter;
+                      // if I have a callback, push the new topic back up to
+                      // lastTopic
+                      if (ctx._callback) {
+                          lastTopic = topic = ctx.emitter;
+                      } else {
+                          topic = ctx.emitter;
+                      }
                 }
             }
 

test/vows-test.js

@@ -484,3 +484,31 @@ vows.describe("Vows with asynchonous teardowns").addBatch({
         }
     }
 }).export(module);
+
+vows.describe('Async topic is passed to vows with topic-less subcontext').addBatch({
+    'Async 42': {
+        topic: function () {
+            var callback = this.callback;
+            process.nextTick(function () {
+                callback(null, 42);
+            });
+        },
+        'equals 42': function (topic) {
+            assert.equal(topic, 42);
+        },
+        'has the property that': {
+            'it is equal to 42': function (topic) {
+                // <-- This vow fails, topic is undefined!?
+                assert.equal(topic, 42);
+            }
+        },
+        'plus 1': {
+            topic: function (parentTopic) {
+                return parentTopic + 1;
+            },
+            'equals 43': function (topic) {
+                assert.equal(topic, 43);
+            }
+        }
+    }
+})['export'](module);