Add test for python-software-properties package

This test adds the precondition that the python-software-packages package be
installed before the apt class is synced. If the defined function were not
called around the package resource, this test would fail with a duplicate
package resource error.

spec/classes/apt_spec.rb

@@ -124,4 +124,9 @@ describe 'apt', :type => :class do
       end
     end
   end
+
+  describe "it should not error if package['python-software-properties'] is already defined" do
+    let(:pre_condition) { 'package { "python-software-properties": }->Class["Apt"]' }
+    it { should contain_package("python-software-properties") }
+  end
 end