Explicitly unfreeze interpolated strings

This fixes a FrozenError on Ruby 2.6/2.7. On Ruby 3.0+ interpolated
strings are always unfrozen

Author: jhawthorn

lib/rexml/element.rb

@@ -356,7 +356,7 @@ def initialize( arg = UNDEFINED, parent=nil, context=nil )
     #   e.inspect # => "<foo bar='0' baz='1'> ... </>"
     #
     def inspect
-      rv = "<#@expanded_name"
+      rv = +"<#@expanded_name"
 
       @attributes.each_attribute do |attr|
         rv << " "

lib/rexml/xmldecl.rb

@@ -117,7 +117,7 @@ def content(enc)
         quote = "'"
       end
 
-      rv = "version=#{quote}#{@version}#{quote}"
+      rv = +"version=#{quote}#{@version}#{quote}"
       if @writeencoding or enc !~ /\Autf-8\z/i
         rv << " encoding=#{quote}#{enc}#{quote}"
       end