Fix missing attributes in some events

[ricardograca]

• Related Issues: attrs in 'creating' event handler only contains 'created_at' and 'updated_at' #1433

Introduction

Fixes an issue that prevented all of the attributes that are going to be inserted or updated to be present in the second argument to the event listener of both the creating and updating events.

Motivation

Fixes #1433.

Proposed solution

This simply moves the logic that selects which attributes are going to be saved a bit further up and uses that as the argument to the event handlers, instead of always passing the attrs object.

Also adds another test to verify that save() will use both the attributes passed to it as well as any others that were already set on the model.

[ricardograca]

This was released in version 0.15.0.