The :xml plugin adds the ability of encoding nokogiri objects into request payload (via the :xml param), and parsing a response payload into a nokogiri document.
As a consequence, it depends on the nokogiri gem.
payload = Nokogiri::XML("<xml....")
http = HTTPX.plugin(:xml)
response = http.post(xml_uri, xml: payload)
response.xml #=> returns a Nokogiri::XML object
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