Re-merge the permission with the same value

`#reverse_merge` did not work as planned because :default key was
already existed in the hash.

lib/paperclip/storage/s3.rb

@@ -249,8 +249,8 @@ module Paperclip
       end
 
       def set_permissions permissions
-        permissions = { :default => permissions } unless permissions.respond_to?(:reverse_merge)
-        permissions.reverse_merge :default => :public_read
+        permissions = { :default => permissions } unless permissions.respond_to?(:merge)
+        permissions.merge :default => (permissions[:default] || :public_read)
       end
 
       def parse_credentials creds