MongoDB adapter doesn't recognise FKs to mysql table correctly

**Sails version**: sails@1.0.1
**Node version**: 8.9.4
**NPM version**: 5.6.0
**DB adapter name**: sails-mongo
**DB adapter version**: sails-mongo@1.0.0
**Operating system**: macOS 10.13.3

Hello,

I have a reference from my mongo model ( `Client` ) to a mysql model via `user: { model: 'User', required: true }` and `User` is in mysql database. I then want to create a `Client` and supply `{user: 1}` the adapter throws

`Error: Invalid replacement foreign key value provided for association (`user`). Cannot interpret `1` as a Mongo id.`

REPO: git@bitbucket.org:mp3por/sails-bug-2.git

Please advice :)

Start by reproducing the Client-to-User association described in the issue with sails-mongo@1.0.0 and a MySQL User model, using the linked reproduction repository. Trace the adapter's foreign-key replacement handling when creating Client with {user: 1}. Done means the MongoDB adapter recognises the MySQL foreign key and creates the Client without the Mongo id error.

Written by the indexing model from the issue text.