Added failing tests for '<<' and '<<<some html'.

I called:
```ruby
doc = Nokogiri::XML::Document.parse('<<<some html')
doc.to_s
```
I expected getting the string back. However, Nokogiri returns an empty string.

In the case of '<<', this is what the test returns:
`+"<?xml version=\"<<<some html>\"?>
+"
`

// @rafaelfranca