Fix lens filename in postfix::canonical

Also adds an acceptance test to verify it actually works.

Fixes: 2d8880c ("add support for generating canonical maps")

manifests/canonical.pp

@@ -66,7 +66,7 @@
 
   augeas { "Postfix canonical - ${name}":
     incl    => $_file,
-    lens    => 'Postfix_Canonical.lns',
+    lens    => 'postfix_canonical.lns',
     changes => $changes,
     require => Package['postfix'],
     notify  => Exec["generate ${_file}.${lookup_table_suffix}"],

spec/acceptance/canonical_spec.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+require 'spec_helper_acceptance'
+
+describe 'postfix::canonical' do
+  it_behaves_like 'an idempotent resource' do
+    let(:manifest) do
+      <<~PUPPET
+        include postfix
+        postfix::hash { '/etc/postfix/recipient_canonical':
+          ensure => present,
+        }
+        postfix::config { 'canonical_alias_maps':
+          value => 'hash:/etc/postfix/recipient_canonical',
+        }
+        postfix::canonical { '[email protected]':
+          file        => '/etc/postfix/recipient_canonical',
+          ensure      => present,
+          destination => 'root',
+        }
+      PUPPET
+    end
+  end
+end