Fixed a subtle bug with regards to the Promise.

src/lib.rs

@@ -565,8 +565,9 @@ impl<T> Promise<T> {
 	
 	/// Writes the result value and reawakens the caller.
 	pub fn fulfill(&self, result: T) {
-		self.result.replace(Some(result));
-		self.caller.wake_by_ref();
+		if self.result.replace(Some(result)).is_none() {
+			self.caller.wake_by_ref();
+		}
 	}
 	
 	/// Extracts the written value and resets the state of the promise.