UniqueObject Validators doesn't work if you give the column a different name, than the fields name.

UniqueObject Validators doesn't work if you give the column a different name, than the fields name.

So, if you have an user entity with id as identifier and name the column like 'user_id' (so, the webform also contains a field names 'user_id' the validator will fail at`

``` php
if (!isset($context[$columnName])) {
```

because, there is no 'id' (but 'user_id') in the form.

So, as renaming columns is a supported behaviour shouldn't the UniqueObject validator rely on the choosen/given columnName?

If have a working example here, but don't know, if it is correct in all ways...
Could make a pull request for that.