Update base_contact.py

context is now received as a frozendict, so pop is not allowed on it directly.
We need to make a copy of it first, so 

    if context is None:
        context = {}

is not enought...

base_contact/base_contact.py

@@ -59,8 +59,7 @@ class res_partner(orm.Model):
         Keeping it in context can result in unexpected behaviour (ex: reading
         one2many might return wrong result - i.e with "attached contact" removed
         even if it's directly linked to a company). """
-        if context is None:
-            context = {}
+        context = dict(context or {})
         if mode != 'search':
             context.pop('search_show_all_positions', None)
         return context