Add getValues method for fetching array of docs from collection

[craigbeck]

Found a pattern of using filter with a null filter function in production code that was used to get an array of collection values e.g.

const users = model.filter('users', null).get() || [];

This pattern is unintuitive and highlighted missing functionality to get the collection values.

getValues returns an array of docs, and ensures an empty array when no values at path.