Fix audio always being re-encoded even for passthrough-eligible media (#1839)

lib/paperclip/transcoder.rb

@@ -40,8 +40,10 @@ module Paperclip
         @output_options['f']       = 'image2'
         @output_options['vframes'] = 1
       when 'mp4'
-        @output_options['acodec'] = 'aac'
-        @output_options['strict'] = 'experimental'
+        unless eligible_to_passthrough?(metadata)
+          @output_options['acodec'] = 'aac'
+          @output_options['strict'] = 'experimental'
+        end
 
         if high_vfr?(metadata) && !eligible_to_passthrough?(metadata)
           @output_options['vsync'] = 'vfr'