Simplify NodeSet#to_a with a minor speed-up.

[larskanis]

This stood up while reviewing #1362 (comment) . The speed-up is around 10%:

xml = Nokogiri::XML.parse("<r>#{"<a/>"*100}</r>")
set = xml.xpath('//a')
st = Time.now; 1000000.times{ set.to_a }; Time.now - st

... changed from 8.2 to 7.3 seconds on my (slow) netbook.

[flavorjones]

Great, thanks for following up and making this change!