current-thread: fix shutdown on idle (#763)

When spawning using `Handle` while on the executor, tasks were being
double counted. This prevented the number of active tasks to reach zero,
thus preventing the executor from shutting down.

This changes `spawn` to check if being called from the executor
**before** incrementing the number of active tasks.

Fixes #760

tokio-current-thread/src/lib.rs

@@ -653,6 +653,13 @@ impl Handle {
     where
         F: Future<Item = (), Error = ()> + Send + 'static,
     {
+        if thread::current().id() == self.thread {
+            let mut e = TaskExecutor::current();
+            if e.id() == Some(self.id) {
+                return e.spawn_local(Box::new(future));
+            }
+        }
+
         if self.shut_down.get() {
             return Err(SpawnError::shutdown());
         }
@@ -669,13 +676,6 @@ impl Handle {
             return Err(SpawnError::shutdown());
         }
 
-        if thread::current().id() == self.thread {
-            let mut e = TaskExecutor::current();
-            if e.id() == Some(self.id) {
-                return e.spawn_local(Box::new(future));
-            }
-        }
-
         self.sender.send(Box::new(future))
             .expect("CurrentThread does not exist anymore");
         // use 0 for the id, CurrentThread does not make use of it