sources format and ensure => absent fails

```puppet
apt::source{
  ensure        => absent,
  source_format => sources,
```

produces a compile error every time.

```
    Puppet::PreformattedError:
       Evaluation Error: Error while evaluating a Resource Statement, Apt::Setting[sources-my_source]: parameter 'content' expects a value of type Undef or String[1], got String (file: /home/steve/GIT/puppetlabs-apt/spec/fixtures/modules/apt/manifests/source.pp, line: 363) on node legion.lan
```

Author: traylenator

manifests/source.pp

@@ -348,8 +348,8 @@
           )
         }
         'absent': {
-          $header = undef
-          $source_content = undef
+          $header = '# no content not permitted, ignored for absent case'
+          $source_content = '# not content not permitted, ignored for absent case'
         }
         default: {
           fail('Unexpected value for $ensure parameter.')

spec/defines/source_spec.rb

@@ -455,6 +455,20 @@
       it { is_expected.to contain_apt__setting("sources-#{title}").with_notify_update(true) }
     end
 
+    context 'absent deb822 source' do
+      let :params do
+        super().merge(
+          {
+            ensure: 'absent',
+            location: ['http://debian.mirror.iweb.ca/debian/'],
+            repos: ['main', 'contrib', 'non-free']
+          },
+        )
+      end
+
+      it { is_expected.to contain_apt__setting("sources-#{title}").with_ensure('absent') }
+    end
+
     context 'complex deb822 source' do
       let :params do
         super().merge(