From: nic <user@.../>
Date: 2011-01-11T17:30:27+09:00
Subject: Re: REXML Attribute and Quotation Control

That is hardcoded in the XMLDecl class. From source:

private
		def content(enc)
			rv = "version='#@version'"
			rv << " encoding='#{enc}'" if @writeencoding || enc !~ /utf-8/i
			rv << " standalone='#@standalone'" if @standalone
			rv
		end


You could use the similar code that is suggested in the second answer of the SO post to replace the content() method.