Add to_s as an alias to the to_xml method

lib/oga/xml/to_xml.rb

@@ -7,6 +7,8 @@ module Oga
       def to_xml
         Generator.new(self).to_xml
       end
+
+      alias_method :to_s, :to_xml
     end
   end
 end

spec/oga/xml/to_xml_spec.rb

@@ -0,0 +1,11 @@
+require 'spec_helper'
+
+describe Oga::XML::ToXML do
+  describe '#to_s' do
+    it 'is an alias of to_xml' do
+      node = Oga::XML::Element.new(name: 'foo')
+
+      expect(node.method(:to_s)).to eq(node.method(:to_xml))
+    end
+  end
+end