Fix missing reference

Author: rastating

spec/wordpress/xss_spec.rb

@@ -1,4 +1,5 @@
 require_relative '../spec_helper'
+require 'erb'
 
 describe Wpxf::WordPress::Xss do
   let(:subject) do
@@ -70,7 +71,7 @@
 
   describe '#xss_url_and_ascii_encoded_include_script' do
     it 'returns a URL encoded version of #xss_ascii_encoded_include_script' do
-      expect(subject.xss_url_and_ascii_encoded_include_script).to eq url_encode(subject.xss_ascii_encoded_include_script)
+      expect(subject.xss_url_and_ascii_encoded_include_script).to eq ERB::Util.url_encode(subject.xss_ascii_encoded_include_script)
     end
   end
 end