[FIX] Error when importing customers #3659

Since ir_fields is written with the new API, when importing a partner record with country/state name (not id), it will call the 'name_search' function with the new arguments, which ommit cr, uid, ids, context.

Fixes #3659
Which doesn't work in that case, the conversion is plain broken (there's no `extend` method on recordsets). Merging recordsets [is done with the `|` operator](https://www.odoo.com/documentation/8.0/reference/orm.html#set-operations).