String#end_with? should be faster than Regexp here

actionpack/lib/action_dispatch/testing/assertions.rb

@@ -12,7 +12,7 @@ module Assertions
     include Rails::Dom::Testing::Assertions
 
     def html_document
-      @html_document ||= if @response.content_type.to_s =~ /xml\z/
+      @html_document ||= if @response.content_type.to_s.end_with?("xml")
         Nokogiri::XML::Document.parse(@response.body)
       else
         Nokogiri::HTML::Document.parse(@response.body)