🐛Fixed updated_at not being updated

[kirrg001]

closes #9520

• it contains a dependency bump of the latest Bookshelf release
• Bookshelf introduced a bug in the last release
  • see Use created_at from query if passed bookshelf/bookshelf#1583
  • see Clean up automatic timestamps feature bookshelf/bookshelf#1798
• this has caused trouble in Ghost
  • the updated_at attribute was not automatically set anymore

---

The bookshelf added one breaking change: it's allow to pass custom updated_at and created_at.
We already have a protection for not being able to override the created_at date on update.
We had to add another protection to now allow to only change the updated_at property.
You can only change updated_at if you actually change something else e.g. the title of a post.

To be able to implement this check i discovered that Bookshelfs model.changed object has a tricky behaviour.
It remembers all attributes, which where changed, doesn't matter if they are valid or invalid model properties.
We had to add a line of code to avoid remembering none valid model attributes in this object.

e.g. you change tag.parent (no valid model attribute). The valid property is tag.parent_id.
If you pass tag.parent but the value has not changed (tag.parent === tag.parent_id), it will output you tag.changed.parent. But this is wrong.
Bookshelf detects changed attributes too early. Or if you think the other way around, Ghost detects valid attributes too late.
But the current earliest possible stage is the onSaving event, there is no earlier way to pick valid attributes (except of .forge, but we don't use this fn ATM).
Later: the API should transform tag.parent into tag.parent_id, but we are not using it ATM, so no need to pre-optimise.
The API already transforms post.author into post.author_id.