Compare XML comments

Without this patch, this happens:
```ruby
require 'nokogiri/diff'

xml = "<!-- Foo -->"
doc = Nokogiri::XML(xml)
doc2 = Nokogiri::XML(xml)

doc.diff(doc2) do |change, node|
  puts "#{change} #{node.inspect}"
end
```

```
- #<Nokogiri::XML::Comment:0x3fe3ca11b3fc " Foo ">
+ #<Nokogiri::XML::Comment:0x3fe3ca11b370 " Foo ">
```

After the patch, it recognizes that both documents are the same.