Stop sources format and ensure => absent failing

Currently  the following produces a compile error every time.

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

```
    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

@@ -360,9 +360,14 @@
       fail("Unexpected APT source format: ${source_format}")
     }
   }
+
+  $_combined = ($header or $source_content) ? {
+    true    => "${header}${source_content}",
+    default => undef,
+  }
   apt::setting { "${_file_suffix}-${name}":
     ensure        => $ensure,
-    content       => "${header}${source_content}",
+    content       => $_combined,
     notify_update => $notify_update,
   }
 }

spec/defines/source_spec.rb

@@ -516,5 +516,36 @@
 
       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',
+          },
+        )
+      end
+
+      it { is_expected.to contain_apt__setting("sources-#{title}").with_ensure('absent') }
+    end
+
+    context 'absent complex deb822 source' do
+      let :params do
+        super().merge(
+          {
+            ensure: 'absent',
+            types: ['deb', 'deb-src'],
+            location: ['http://fr.debian.org/debian', 'http://de.debian.org/debian'],
+            release: ['stable', 'stable-updates', 'stable-backports'],
+            repos: ['main', 'contrib', 'non-free'],
+            architecture: ['amd64', 'i386'],
+            allow_unsigned: true,
+            notify_update: false
+          },
+        )
+      end
+
+      it { is_expected.to contain_apt__setting("sources-#{title}").with_ensure('absent') }
+    end
   end
 end