make backports_url follow debian_url by default

backports_url was already set to the same value than debian_url by
default, but it was set to the default value of debian_url. this meant
that when giving a url to the debian_url parameter to the apt class, the
backports_url would not follow and would still be using the default
value for debian_url.

with this change, when backports_url is not specified, but debian_url
is, then both of them take on the value given to the debian_url. of
course if backports_url is also given a value, then it keeps the value
given by the user.

manifests/init.pp

@@ -35,6 +35,11 @@ class apt(
     }
   }
 
+  $real_backports_url = $backports_url ? {
+    false   => $debian_url,
+    default => $backports_url,
+  }
+
   package { 'apt':
     ensure  => installed,
     require => undef,
@@ -99,12 +104,12 @@ class apt(
   if ($use_backports and !($::debian_release in ['testing', 'unstable', 'experimental'])) {
     apt::sources_list {
       'backports':
-        content => "deb $backports_url ${::debian_codename}-backports ${apt::real_repos}",
+        content => "deb ${real_backports_url} ${::debian_codename}-backports ${apt::repos}",
     }
     if $include_src {
       apt::sources_list {
         'backports-src':
-          content => "deb-src $backports_url ${::debian_codename}-backports ${apt::real_repos}",
+          content => "deb-src ${real_backports_url} ${::debian_codename}-backports ${apt::repos}",
       }
     }
   }

manifests/params.pp

@@ -9,10 +9,7 @@ class apt::params () {
   $ubuntu_url = 'http://archive.ubuntu.com/ubuntu'
   $backports_url = $::debian_codename ? {
     'squeeze'  => 'http://backports.debian.org/debian-backports/',
-    default => $::operatingsystem ? {
-      'Ubuntu' => $ubuntu_url,
-      default  => $debian_url,
-    }
+    default => false,
   }
   $lts_url = $debian_url
   $volatile_url = 'http://volatile.debian.org/debian-volatile/'