Add spec for desired behavior
user
```ruby
require 'rails_helper'

RSpec.describe "get a 404 response (in JSON) from unmatched route", :type => :request do
  it 'when the request id for HTML' do
    get '/foo'
    expect(response.body).to eq('{"error":"404 - Not Found"}')
    expect(response).to have_http_status(:not_found)
  end
  
  it 'when the request id for CSV' do
    get '/foo', headers: {"Content-Type": "text/csv"}
    expect(response.body).to eq('{"error":"404 - Not Found"}')
    expect(response).to have_http_status(:not_found)
  end

  it 'when the request id for JSON' do
    get '/foo', headers: {"Content-Type": "application/vnd.uktt.v2"}
    expect(response.body).to eq('{"error":"404 - Not Found"}')
    expect(response).to have_http_status(:not_found)
  end
end
```
assistant