Add spec for subdir mounting

spec/config_spec.rb

@@ -0,0 +1,32 @@
+require "spec_helper"
+
+describe "Rack config" do
+  include Rack::Test::Methods
+  let(:app) {
+    Rack::Builder.parse_file('config.ru').first
+  }
+  describe "when RAILS_RELATIVE_URL_ROOT is absent" do
+    it "mounts the app to '/'" do
+      get '/'
+      expect(last_response).to be_ok
+    end
+  end
+
+  describe "when RAILS_RELATIVE_URL_ROOT is present" do
+    let(:new_root) { "/foobar" }
+
+    before do
+      @root = ENV["RAILS_RELATIVE_URL_ROOT"]
+      ENV["RAILS_RELATIVE_URL_ROOT"] = new_root
+    end
+
+    after do
+      ENV["RAILS_RELATIVE_URL_ROOT"] = @root
+    end
+
+    it "mounts the app to the subdirectory" do
+      get new_root
+      expect(last_response).to be_ok
+    end
+  end
+end