[FIX] auth_signup: don't crash on user copy

Altering a frozen dict is not allowed

Backport from 7c513773e9

addons/auth_signup/models/res_users.py

@@ -149,8 +149,8 @@ class ResUsers(models.Model):
     @api.multi
     def copy(self, default=None):
         self.ensure_one()
-        context = self.env.context
+        sup = super(ResUsers, self)
         if not default or not default.get('email'):
             # avoid sending email to the user we are duplicating
-            context['reset_password'] = False
-        return super(ResUsers, self.with_context(context)).copy(default=default)
+            sup = super(ResUsers, self.with_context(reset_password=False))
+        return sup.copy(default=default)