Fixed only_process option with proc (#2289)

Bug introduced in 1c7d7f66

Per https://github.com/thoughtbot/paperclip/commit/1c7d7f66ceea27cfe604e3a13c2eb87df217d13a#commitcomment-18765681 request

lib/paperclip/attachment.rb

@@ -240,7 +240,8 @@ module Paperclip
     # the instance's errors and returns false, cancelling the save.
     def save
       flush_deletes unless @options[:keep_old_files]
-      if @options[:only_process].any? && !@options[:only_process].include?(:original)
+      process = only_process
+      if process.any? && !process.include?(:original)
         @queued_for_write.except!(:original)
       end
       flush_writes

spec/paperclip/attachment_spec.rb

@@ -500,6 +500,7 @@ describe Paperclip::Attachment do
       @attachment.expects(:post_process).with(:thumb)
       @attachment.expects(:post_process).with(:large).never
       @attachment.assign(@file)
+      @attachment.save
     end
   end