Rails route is not found in the tests

This application is a Rails 4 app and just an API (at this point). I can hit my URL from the browser, but when I try to access it in a test it can't find the URL. I get this:

No route matches {:action=>"/api/v1/users/20", :controller=>"api/v1/users"}

There are not any assertions in my test yet. Just trying to get past this error first:

# /spec/controllers/api/v1/users_controller_spec.rb
require 'rails_helper'

RSpec.describe Api::V1::UsersController, :type => :controller do
  describe "User API" do
    it "can return a user by ID" do
      user = FactoryGirl.create(:user)

      get "/api/v1/users/#{user.id}"
    end
  end
end

And my controller:

# app/controllers/api/v1/users_controller.rb
class Api::V1::UsersController < ApplicationController
  before_action :set_user, only: [:show]

  def show 
  end

  private

  def set_user
    @user = User.find(params[:id])
  end
end

Any my routes:

# config/routes.rb
Rails.application.routes.draw do
  namespace :api, defaults: {format: 'json'} do
    namespace :v1 do
      resources :users, only: [:show]
    end
  end
end

And rake routes give me:

     Prefix Verb URI Pattern                 Controller#Action
api_v1_user GET  /api/v1/users/:id(.:format) api/v1/users#show {:format=>"json"}

And my gems:

group :test do
  gem 'capybara'
end

group :development, :test do
  gem 'rspec-rails'
  gem 'factory_girl_rails'
  gem 'database_cleaner'
end

I'm sure there is something simple I am missing here, but I've spent a couple hours and can't figure it out.