Fixes postmap when ensure=absent (#202)

Fixes an issue where if a postfix map $ensure is 'absent' the postmap exec resource causes puppet runs to fail.

manifests/map.pp

@@ -80,8 +80,13 @@ define postfix::map (
     }
   }
 
+  $generate_cmd = $ensure ? {
+    'absent'  => "rm ${path}.db",
+    'present' => "postmap ${path}",
+  }
+
   exec {"generate ${name}.db":
-    command     => "postmap ${path}",
+    command     => $generate_cmd,
     path        => $::path,
     #creates    => "${name}.db", # this prevents postmap from being run !
     refreshonly => true,

spec/defines/postfix_map_spec.rb

@@ -110,7 +110,7 @@ describe 'postfix::map' do
         it { is_expected.to contain_file('postfix map foo').with_ensure('absent') }
         it { is_expected.to contain_file('postfix map foo').without_notify }
         it { is_expected.to contain_file('postfix map foo.db').with_ensure('absent') }
-        it { is_expected.to contain_exec('generate foo.db') }
+	it { is_expected.to contain_exec('generate foo.db').with(:command => 'rm /etc/postfix/foo.db') }
       end
 
       context 'when using pcre type' do