rails: Turning on jbuilder for API only apps cause controller test failures

Steps to replicate:

rails new robots --api
cd robots
rails generate scaffold post title:string
rails db:migrate
<Uncomment jbuilder gem from Gemfile>
bundle
rails test

Failures:

PostsControllerTest#test_should_update_post:
ActionView::MissingTemplate: Missing template posts/update, application/update with {:locale=>[:en], :formats=>[:html], :variants=>[], :handlers=>[:raw, :erb, :html, :builder, :ruby, :jbuilder]}. Searched in:
  * "/Users/david/Code/demos/robots/app/views"

    app/controllers/posts_controller.rb:30:in `update'
    test/controllers/posts_controller_test.rb:27:in `block in <class:PostsControllerTest>'

Line 30 has render json: @post, so its not relying on default rendering. It should just be calling #to_json on @post and return that. Shouldn’t matter what rendering engines are installed or not.