Fix TagList formwidget

[mjauvin]

The TagList formwidget was not working properly in relation mode if the relation was not using a pivot table (e.g. morphMany/hasMany)

All modes did not update the model field or relation when all tags had been cleared.

Fixes #1223

[mjauvin]

@bennothommo I just saw you started a PR on TagList as well, thought you might want to review this one.

[bennothommo]

@mjauvin it's probably worth adding an example in the Test plugin if there isn't one already, if adding the unit tests becomes too difficult.

[mjauvin]

yeah, that's what I'm going to do for now.

[mjauvin]

tested working with the following modes using Winter.Test plugin in Post & Record models:

• relation (belongsToMany, hasMany, morphMany)
• array (useKey = true/false)
• string (useKey = true/false)

All the above tests also include clearing all values and making sure the model field or relation was updated accordingly.

[mjauvin]

@LukeTowers Anything more before merging this?

[LukeTowers]

@mjauvin @bennothommo Has it been fully tested?

[mjauvin]

@mjauvin @bennothommo Has it been fully tested?

Yes, see previous comment above.