Feature/merchant index plus tests

[litobot]

• Can return all merchants

• Can sort merchants by created_at time newest to oldest

• Can retrieve merchants whose invoices have been "returned" ^ All these tests passing

• Count functionality missing (Coming soon!)

[jimmacur]

Hey Lito, great work on this! I see that you’ve refined the merchants’ index action to handle sorting and filtering more flexibly, and it looks like the new methods in the Merchant model are handling those cases well. The test cases for sorting by creation date and filtering by returned items are also well-covered. Nice job integrating these changes and updating the serializer. Everything looks good to me!

I also see that you've got some code setup and ready for some features in the future! You did SO MUCH WORK HERE!!

[litobot]

Okay. I can't let so much time go by without merging. I think everything should be fine. There might be a missing end here or there, but we'll find out right now.