Implement construct data for datetime + bugfix in datahandler base class

- datahandler base class was not supporting default values due to a bug
- implements the datetime datahandler construct_data method

lodel/leapi/datahandlers/datas_base.py

@@ -1,5 +1,6 @@
 #-*- coding: utf-8 -*-
 import warnings
+import datetime
 
 from lodel.leapi.datahandlers.base_classes import DataField
 
@@ -89,6 +90,10 @@ class DateTime(DataField):
         error = None
         return value, error
 
+    def construct_data(self, emcomponent, fname, datas, cur_value):
+        if (self.now_on_create and cur_value is None) or self.now_on_update:
+            return datetime.datetime.now()
+
 ##@brief Data field designed to handle long string
 class Text(DataField):
     help = 'A text field (big string)'