add invitation_only field to course serializer

lms/djangoapps/course_api/serializers.py

@@ -73,6 +73,7 @@ class CourseSerializer(serializers.Serializer):  # pylint: disable=abstract-meth
     pacing = serializers.CharField()
     mobile_available = serializers.BooleanField()
     hidden = serializers.SerializerMethodField()
+    invitation_only = serializers.BooleanField()
 
     # 'course_id' is a deprecated field, please use 'id' instead.
     course_id = serializers.CharField(source='id', read_only=True)

lms/djangoapps/course_api/tests/test_serializers.py

@@ -72,6 +72,7 @@ class TestCourseSerializer(CourseApiFactoryMixin, ModuleStoreTestCase):
             'pacing': 'instructor',
             'mobile_available': False,
             'hidden': False,
+            'invitation_only': False,
 
             # 'course_id' is a deprecated field, please use 'id' instead.
             'course_id': u'edX/toy/2012_Fall',