Workaround odd OpenStruct issue

Due to differences in Ruby 2.7/3.0 The field isn't correctly initialised
otherwise.

lib/dough/html_options.rb

@@ -11,6 +11,10 @@ def method_missing(m, *args, &block)
       super
     end
 
+    def classes
+      self[:classes] ||= ''
+    end
+
     def to_s
       hash = table.dup
       hash[:class] = hash.delete(:classes)

spec/lib/dough/html_options_spec.rb

@@ -1,6 +1,12 @@
 require 'spec_helper'
 
 describe Dough::HtmlOptions do
+  describe '#classes' do
+    it 'defaults to an empty string' do
+      expect(described_class.new.classes).to eq('')
+    end
+  end
+
   describe 'to_s' do
     subject { described_class.new(id: 'a', classes: 'b c') }