patchbomb: normalize date format in generated mboxes
mbox format should use time.asctime(). Unfortunately, this function writes
2-characters day of week on Windows while unix one writes a single character.
Normalize to Windows version since the other one can hardly be written with
strftime().
<item>
<title>{tag|escape}</title>
<link>{urlbase}{url}rev/{node|short}</link>
<description><![CDATA[{tag|strip|escape|addbreaks}]]></description>
<pubDate>{date|rfc822date}</pubDate>
</item>