add created datetime in comment schema

tracim/tests/functional/test_comments.py

         # TODO - G.M - 2018-06-172 - [avatar] setup avatar url
         assert comment['author']['avatar_url'] == None
         assert comment['author']['public_name'] == 'Bob i.'
+        # TODO - G.M - 2018-06-179 - better check for datetime
+        assert comment['created']
 
         comment = res.json_body[2]
         assert comment['content_id'] == 20
         # TODO - G.M - 2018-06-172 - [avatar] setup avatar url
         assert comment['author']['avatar_url'] == None
         assert comment['author']['public_name'] == 'John Reader'
+        # TODO - G.M - 2018-06-179 - better check for datetime
+        assert comment['created']
 
     def test_api__post_content_comment__ok_200__nominal_case(self) -> None:
         """
         # TODO - G.M - 2018-06-172 - [avatar] setup avatar url
         assert comment['author']['avatar_url'] is None
         assert comment['author']['public_name'] == 'Global manager'
+        # TODO - G.M - 2018-06-179 - better check for datetime
+        assert comment['created']
 
         res = self.testapp.get('/api/v2/workspaces/2/contents/7/comments', status=200)  # nopep8
         assert len(res.json_body) == 4
         # TODO - G.M - 2018-06-172 - [avatar] setup avatar url
         assert comment['author']['avatar_url'] is None
         assert comment['author']['public_name'] == 'Global manager'
+        # TODO - G.M - 2018-06-179 - better check for datetime
+        assert comment['created']
 
         res = self.testapp.delete(
             '/api/v2/workspaces/2/contents/7/comments/18',
         # TODO - G.M - 2018-06-172 - [avatar] setup avatar url
         assert comment['author']['avatar_url'] is None
         assert comment['author']['public_name'] == 'Bob i.'
+        # TODO - G.M - 2018-06-179 - better check for datetime
+        assert comment['created']
 
         res = self.testapp.delete(
             '/api/v2/workspaces/2/contents/7/comments/19',
         # TODO - G.M - 2018-06-172 - [avatar] setup avatar url
         assert comment['author']['avatar_url'] is None
         assert comment['author']['public_name'] == 'Bob i.'
+        # TODO - G.M - 2018-06-179 - better check for datetime
+        assert comment['created']
 
         res = self.testapp.delete(
             '/api/v2/workspaces/2/contents/7/comments/19',
         # TODO - G.M - 2018-06-172 - [avatar] setup avatar url
         assert comment['author']['avatar_url'] is None
         assert comment['author']['public_name'] == 'John Reader'
+        # TODO - G.M - 2018-06-179 - better check for datetime
+        assert comment['created']
 
         res = self.testapp.delete(
             '/api/v2/workspaces/2/contents/7/comments/20',
         # TODO - G.M - 2018-06-172 - [avatar] setup avatar url
         assert comment['author']['avatar_url'] is None
         assert comment['author']['public_name'] == 'John Reader'
+        # TODO - G.M - 2018-06-179 - better check for datetime
+        assert comment['created']
 
         res = self.testapp.delete(
             '/api/v2/workspaces/2/contents/7/comments/20',
         # TODO - G.M - 2018-06-172 - [avatar] setup avatar url
         assert comment['author']['avatar_url'] is None
         assert comment['author']['public_name'] == 'John Reader'
+        # TODO - G.M - 2018-06-179 - better check for datetime
+        assert comment['created']
 
         res = self.testapp.delete(
             '/api/v2/workspaces/2/contents/7/comments/20',

tracim/views/core_api/schemas.py

         example='<p>This is just an html comment !</p>'
     )
     author = marshmallow.fields.Nested(UserDigestSchema)
+    created = marshmallow.fields.DateTime(
+        format='%Y-%m-%dT%H:%M:%SZ',
+        description='comment creation date',
+    )
 
 
 class ContentModifySchema(marshmallow.Schema):