Sub-model references are not instantiated on first `get`.

[Jule-]

I'm submitting a bug report

• Framework Version:
  v0.4.9

• Language:
  ESNext

Current behavior:
When first get request is processed, models are not well processed and then referenced sub-models are not created.

Expected/desired behavior:
When requiring model for the first time we want sub-models to be well instantiated.

---

In fact issue seems to be somewhere related to the backendKey to frontendKey conversion.

aurelia-collection/src/collection.js

Line 317 in c65917e

let itemData = model[item.backendKey];

Indeed here we are trying to get in model the backendKey while inside it is already converted to frontendKey. As a result itemData is undefined and sub-models will not be instantiated.