Don't close IO object in UploadedFile#open if it's already closed

It seems that the StringIO object (and maybe some other IO objects as
well) raise an "IOError: closed stream" when attempting to close them if
they were already closed on older Ruby versions (2.2 and below).

lib/shrine.rb

@@ -808,7 +808,7 @@ def mime_type
         #
         #     uploaded_file.open { |io| io.read } # the IO is automatically closed
         def open(*args)
-          @io.close if @io
+          @io.close if @io && !(@io.respond_to?(:closed?) && @io.closed?)
           @io = storage.open(id, *args)
 
           return @io unless block_given?