Fix TaxCategory default not showing up in admin

[vl3]

Description
When visiting New products page, Spree::TaxCategory with is_default set to true is not being shown as the first selected option in the tax category select box. Additionally, the hint of the select box is saying that the default is None despite having other Spree::TaxCategory as default. This PR aims to fix it.

Without change

With change

Checklist:

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message
• I have updated Guides and README accordingly to this change (if needed)
• I have added tests to cover this change (if needed)
• I have attached screenshots to this PR for visual changes (if needed)

[kennyadsl]

Thanks! It seems to make sense but I can't find in the codebase a single place where we are using the default tax category. I mean in the tax calculation... Can you help me understand if it's better to remove the concept of the default tax category entirely?

[jarednorman]

I think it is useful to have a tax category that's the default for creating new products. Beyond that I don't think it has any meaning, but it's still useful for that.

[kennyadsl]

Thanks, I think it's ready to be merged so I'm going to approve.

An extra thing that we could do in terms of UX is explaining what's the meaning of the default checkbox in the Tax Category edit page with another hint:

Of course, we can also do that with another PR. Let me know if you want to take care of that or I'll open a new issue to keep track of this.

Thanks again! 🙏

[kennyadsl]

👍 Thanks!

[aldesantis]

Thanks @vl3, just left a comment on an extra SQL query that I think we can get rid of.

[vl3]

Thanks, I think it's ready to be merged so I'm going to approve.

An extra thing that we could do in terms of UX is explaining what's the meaning of the default checkbox in the Tax Category edit page with another hint:

Of course, we can also do that with another PR. Let me know if you want to take care of that or I'll open a new issue to keep track of this.

Thanks again!

I can take care of that next week. Is it ok if I merge this and then open another PR?

[aldesantis]

@vl3 thanks, looking great! Also looking forward to your next PR. ❤️