Serialize XML node as document with correct namespaces

How can I serialize a XML node taken from a document into a standalone document? Using node.to_xml is not enough, because to_xml does not output all the needed namespaces, only those explicitly declared inside that node.

For example, I have this XML document in doc

 <wrapper xmlns="ns" xmlns:extra="extra">
     <record xml:id="r1">
         <field>aaa</field>
         <field extra:type="second">bbb</field>
     </record>
 </wrapper>

and I want to I isolate the node #r1.

 record = doc.at("//*[@xml:id='r1']")

Now, record.to_xml returns (reformatted)

 <record xml:id="r1">
     <field>aaa</field>
     <field extra:type="second">bbb</field>
 </record>

This result may be OK in some contexts, but it is not good for my purposes: the default namespace and the extra namespace have not been copied. What I want is a document in which all the needed namespaces have been copied, like the following:

 <record xml:id="r1" xmlns="ns" xmlns:extra="extra">
     <field>aaa</field>
     <field extra:type="second">bbb</field>
 </record>

How can I do this with Nokogiri?