[Minor] check for spam flag from United Internet

This header is present for most outgoing messages from United Internet which
includes "web.de" and GMX (freemailers which are very popular in Germany) as
well as messages sent from 1and1 webhosting packages.

rules/regexp/upstream_spam_filters.lua

     description = "Message was already marked as spam",
     group = 'upstream_spam_filters'
 }
+
+reconf['UNITEDINTERNET_SPAM'] = {
+    re = 'X-UI-Out-Filterresults=/^junk:/H',
+    score = 5,
+    description = "United Internet says this message is spam",
+    group = 'upstream_spam_filters'
+}