Allow use of a prefixed namespace on a root node using Nokogiri::XML::Builder

Nokogiri 1.5.7 included code to allow Builder to predefine namespace prefixes the same way XML does, e.g.:

`xml[:meat].bacon(:'xmlns:meat' => 'info:mmm/mmm/good')`

But it didn't work on root nodes. This commit fixes that (and adds a test for it, natch).