Set model.id if attributes contain a parsed version of idAttribute

[ricardograca]

• Related Issues: Fetching using hasMany association not working as expected #818, Models not getting populated in collection when using withRelated for hasMany relationship #957
• Previous PRs: Parse idAttribute to set model.id #1082

Introduction

This ensures that Model#id is set correctly even if the model has a custom parse method that mutates attribute keys.

Motivation

This is a followup to the previous PR that needed some changes but its author didn't respond.

Proposed solution

This is almost identical to the previous PR, except that it doesn't store the parsed id attribute when extending the model but instead parses it when it's actually needed. This ensures that the parsed idAttribute is always correct.

This PR also doesn't include the exception when not all relations can be retrieved since it wasn't clear what the purpose of that code was.

Closes #1082.