I want to omit specifying default namespace using libxml-ruby - ruby

Question

I have questions about libxml-ruby.
There is a xml file "sample.xml".
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<worksheet xmlns="http://***" xmlns:r="http://???">
<sheetData>
<row><v>1</v></row>
</sheetData>
</worksheet>
I want to deal with nodes without specifying default namespace like below.
xml = XML::Document.file('sample.xml')
sheet_data = xml.find_first('sheetData')
Of course, I can do it like below.
NS = {
main: 'http://***',
r: 'http://???',
}
sheet_data = xml.find_first('main:sheetData', NS)
But I want to omit string of default namespace.
I tried some properties and methods belongs to XML::Namespace[s], but not effected.
And one more problem when I save a xml file.
ns = XML::Namespace.new(xml.root, 'main', 'http://***')
row = XML::Node.new('row', nil, ns)
sheet_data << row
xml.save("sample.xml")
Published like below.
<row><v>1</v></row>
<main:row/>
I want that it's omitted string of "main:".
So I do this, but it's really ugly.
open('sample.xml', 'wb') do |f|
f.write(xml.to_s.gsub(/(<\/?)main:/, '\1'))
end
Do you have any good idea?