Include GeneratedCertificate modified date in certificates API response.

lms/djangoapps/certificates/apis/v0/tests/test_views.py

@@ -163,6 +163,7 @@ class CertificatesListRestApiTest(AuthAndScopesTestMixin, SharedModuleStoreTestC
                 'course_organization': self.course.org,
                 'certificate_type': CourseMode.VERIFIED,
                 'created_date': self.now,
+                'modified_date': self.now,
                 'status': CertificateStatuses.downloadable,
                 'is_passing': True,
                 'download_url': 'www.google.com',

lms/djangoapps/certificates/apis/v0/views.py

@@ -227,6 +227,7 @@ class CertificatesListView(GenericAPIView):
                     'course_organization': user_cert.get('course_organization'),
                     'certificate_type': user_cert.get('type'),
                     'created_date': user_cert.get('created'),
+                    'modified_date': user_cert.get('modified'),
                     'status': user_cert.get('status'),
                     'is_passing': user_cert.get('is_passing'),
                     'download_url': user_cert.get('download_url'),