Load the celery.task.http module at init so the http tasks are registered.

celery/task/__init__.py

@@ -10,6 +10,7 @@ from celery.registry import tasks
 from celery.task.base import Task, TaskSet, PeriodicTask, ExecuteRemoteTask
 from celery.task.control import discard_all
 from celery.task.builtins import PingTask
+from celery.task.http import HttpDispatchTask
 
 __all__ = ["Task", "TaskSet", "PeriodicTask", "tasks", "discard_all",
            "dmap", "dmap_async", "execute_remote", "ping"]