Fixed celery task.

ECOM-3437

lms/djangoapps/certificates/signals.py

@@ -5,6 +5,7 @@ from celery.task import task
 from django.dispatch.dispatcher import receiver
 
 from certificates.models import CertificateGenerationCourseSetting
+from opaque_keys.edx.keys import CourseKey
 from xmodule.modulestore.django import SignalHandler, modulestore
 
 
@@ -14,12 +15,13 @@ def _listen_for_course_publish(sender, course_key, **kwargs):  # pylint: disable
     enable the self-generated certificates by default for self-paced
     courses.
     """
-    enable_self_generated_certs.delay(course_key)
+    enable_self_generated_certs.delay(unicode(course_key))
 
 
 @task()
 def enable_self_generated_certs(course_key):
     """Enable the self-generated certificates by default for self-paced courses."""
+    course_key = CourseKey.from_string(course_key)
     course = modulestore().get_course(course_key)
     is_enabled_for_course = CertificateGenerationCourseSetting.is_enabled_for_course(course_key)
     if course.self_paced and not is_enabled_for_course: