Fix a regexp to guess HTML

```
1.9.1 :001 > Nokogiri.parse(open('https://github.com/sparklemotion/nokogiri/commits.atom').read).html?
 => true
1.9.1 :002 > Nokogiri.parse(open('https://github.com/sparklemotion/nokogiri/commits.atom').read).xml?
 => false
```

Atom is not HTML! Guess to HTML when string a "html" is included in a attribute-value.