Ensure on_change arguments are loaded

As proteus is lazy about field it read, after a save call there is no field
read. But if the next on_change need the value of a field no loaded, it will
send an empty value because Eval accessors do not trigger a read. So to behave
like other client (which load displayed fields), we ensure to load the needed
arguments.

issue8450
review264411005

proteus/__init__.py

@@ -1034,6 +1034,16 @@ class Model(object):
         return values
 
     def _on_change_args(self, args):
+        # Ensure arguments has been read
+        for arg in args:
+            record = self
+            for i in arg.split('.'):
+                if i in record._fields:
+                    getattr(record, i)
+                elif i == '_parent_' + record._parent_name:
+                    getattr(record, record._parent_name)
+                    record = record._parent
+
         res = {}
         values = _EvalEnvironment(self, 'on_change')
         for arg in args: