User Story 12, Can't Favorite a Pet More Than Once

[EHale64]

As a visitor
After I've favorited a pet
When I visit that pet's show page
I no longer see a link to favorite that pet
But I see a link to remove that pet from my favorites
When I click that link
A delete request is sent to "/favorites/:pet_id"
And I'm redirected back to that pets show page where I can see a flash message indicating that the pet was removed from my favorites
And I can now see a link to favorite that pet
And I also see that my favorites indicator has decremented by 1