Make sure MUNINCOLLECTOR join() gets an array in munin rule

manifests/rules/munin.pp

 class shorewall::rules::munin(
   $munin_port = '4949',
-  $munin_collector = '127.0.0.1',
+  $munin_collector = ['127.0.0.1'],
   $collector_source = 'net'
 ){
     shorewall::params { 'MUNINPORT': value => $munin_port }
-    shorewall::params { 'MUNINCOLLECTOR': value => join($munin_collector,',') }
+    shorewall::params { 'MUNINCOLLECTOR': value => join(any2array($munin_collector),',') }
     shorewall::rule{'net-me-munin-tcp':
         source          => "${collector_source}:\$MUNINCOLLECTOR",
         destination     => '$FW',