remove DKIM-Signature: from headers.  gmail sets this.

Reference: /n/sources/patch/applied/dkim-ignore
Date: Sun Feb 25 17:37:01 CET 2007
Signed-off-by: quanstro@quanstro.net

--- /mail/lib/ignore	Sun Feb 25 17:36:27 2007
+++ /mail/lib/ignore	Sun Feb 25 17:36:26 2007
@@ -14,3 +14,4 @@
 In-Reply-To:
 Delivered-To:
 DomainKey-Signature:
+DKIM-Signature: