Fix inconsistent timestamp values

[ricardograca]

• Related Issues: #1068, #572
• Previous PRs: #1546

Introduction

Ensure date values of automatic timestamps are always returned in a consistent format.

Motivation

When using SQLite dates are returned as Unix timestamps when fetching data from the database, but when a model is created and saved they are stored as Date objects on the model. This creates inconsistency when dealing with dates between saving and fetching, which leads to users having to deal with this inconsistency themselves. However it should be the ORM's job to deal with these issues and provide a consistent interface for users. See the above related issue for more info.

Fixes #1068.

Proposed solution

This adds a new method to format the dates whenever a model has the hasTimestamps attribute set to true or an array of key names. Unlike the previous PR this doesn't wrap existing methods (parse) and just calls the new method when needed. The advantage of this approach is that the parse method is kept untouched which is a positive thing since it's already complicated and tangled as it is.

Current PR Issues

Doesn't format other date columns the user might have set on the database, but since Bookshelf doesn't know about the database structure it wouldn't be feasible anyway.

There's also a problem in the way that MySQL handles dates that causes millisecond information to be discarded when saving to the database. This can be fixed but requires a change in Knex and there's nothing we can do on our side except for recommending that users use a custom date format in migrations when dealing with MySQL.