[FIX] models: `_convert_to_write()` shall not return `NewId`

The result of _convert_to_write() is intended to be pass directly to
`write()` or returned to the client (`onchange()` and `default_get()`.
`NewId` is as special value that must not be stored into the database
or exposed to the client.
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-win32