code: public Observable<Void> reactivateAsync(String jobId, String taskId) {
          return reactivateWithServiceResponseAsync(jobId, taskId).map(new Func1<ServiceResponseWithHeaders<Void, TaskReactivateHeaders>, Void>() {
              @Override
              public Void call(ServiceResponseWithHeaders<Void, TaskReactivateHeaders> response) {
                  return response.body();
              }
          });
      }
docs: Reactivates a task, allowing it to run again even if its retry count has been exhausted.
  Reactivation makes a task eligible to be retried again up to its maximum retry count. The task's state is changed to active. As the task is no longer in the completed state, any previous exit code or failure information is no longer available after reactivation. Each time a task is reactivated, its retry count is reset to 0. Reactivation will fail for tasks that are not completed or that previously completed successfully (with an exit code of 0). Additionally, it will fail if the job has completed (or is terminating or deleting).
  
  @param jobId The ID of the job containing the task.
  @param taskId The ID of the task to reactivate.
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the {@link ServiceResponseWithHeaders} object if successful.