Using node.default_unless instead of node.default in _attributes

* Wrapper cookbook usually uses default level to override attributes
   of the wrapped cookbook. Here, we had a problem because if the
   wrapper cookbook was enabling sasl and was setting a relayhost
   and a user/password combination using the default level, this was
   not working because _attributes resets those attributes.

   To fix this, we now use node.default_unless which will set the
   attribute only if it's not already set ensuring that wrapper
   cookbook overriden attributes using default level are correctly
   kept when computing derived attributes.

spec/wrapper_spec.rb

@@ -0,0 +1,20 @@
+require 'spec_helper'
+
+##
+# Spec to ensure wrapper cookbook can correctly override
+# attributes using default level without _attributes
+# recipe clearing them.
+
+describe 'wrapper::default' do
+  cached(:chef_run) do
+    ChefSpec::SoloRunner.new.converge(described_recipe)
+  end
+
+  describe '_attributes recipes' do
+    it 'keeps wrapper cookbook default set attributes' do
+      expect(chef_run.node['postfix']['main']['relayhost']).to eq('please')
+      expect(chef_run.node['postfix']['main']['smtp_sasl_security_options']).to eq('keep')
+      expect(chef_run.node['postfix']['sasl']['smtp_sasl_user_name']).to eq('us')
+    end
+  end
+end