[Mimedefang] older version X-Scanned-by header remains after upgrade

[Xiaoyan Ma]

I am upgrading mimedefang from 2.33 to 2.39. One thing I noticed it that 
the older version's X-Scanned-by co-exists with the new one:
X-Scanned-By: MIMEDefang 2.39
X-Scanned-By: MIMEDefang 2.33 (www . roaringpenguin . com / mimedefang)

I was able to remove the 2.33 header by adding 
"action_delete_header("X-Scanned-by")" in the filter, but I am wondering if 
there is something didn't get cleaned up when upgrading.

Thanks.
Xiaoyan