Set role correctly. (#188)

Fixes #187

app/controllers/users_controller.rb

@@ -36,7 +36,7 @@ def create
     @user = User.new(user_params)
 
     if User.none?
-      @user.admin = true
+      @user.role = "admin"
     end
 
     if @user.save

test/integration/users_test.rb

@@ -0,0 +1,21 @@
+require "test_helper"
+
+class UsersTest < ActionDispatch::IntegrationTest
+  test "when no users exist an unauthenticated user can create an admin" do
+    User.delete_all
+
+    assert_difference "User.count" do
+      post users_path, params: { user: FactoryBot.attributes_for(:user) }
+
+      assert_redirected_to root_path
+    end
+
+    assert User.last.admin?
+  end
+
+  test "when users exists an unauthenticated user cannot create any users" do
+    post users_path, params: { user: FactoryBot.attributes_for(:user) }
+
+    assert_redirected_to login_path
+  end
+end