Created by: BrunoBerisso
When a continuation block is executed with a BFExecutor the internal block that makes the actual call check if the continuation block returns a BFTask, if that is the case a call to continueWithBlock: is made and the completion source is updated accordingly to the result of the task.
This pull request add code to check if the returned task is complete and update the completion source without made an extra call to continueWithBlock:. This increase the performance on memory consumption and execution time. And also make sence :)