Improve ModelStorage.copy

* Allow callable in copy default
* Support dotted notation in copy default
* Use setdefault in copy
* Copy default when needed
* Do not set default state in copy
* Use callable and dotted notation in copy default

issue5576
issue7648
review72291002

purchase.py

@@ -67,8 +67,9 @@ class Purchase(metaclass=PoolMeta):
     def copy(cls, purchases, default=None):
         if default is None:
             default = {}
-        default = default.copy()
-        default['invoice_lines_ignored'] = None
+        else:
+            default = default.copy()
+        default.setdefault('invoice_lines_ignored', None)
         return super(Purchase, cls).copy(purchases, default=default)