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.

test/fixtures/cookbooks/wrapper/attributes/default.rb

@@ -0,0 +1,6 @@
+
+default['postfix']['main']['smtp_sasl_auth_enable'] = 'yes'
+
+default['postfix']['main']['relayhost'] = 'please'
+default['postfix']['sasl']['smtp_sasl_user_name'] = 'keep'
+default['postfix']['sasl']['smtp_sasl_passwd'] = 'us'

test/fixtures/cookbooks/wrapper/metadata.rb

@@ -0,0 +1,3 @@
+name 'wrapper'
+version '0.0.1'
+description 'Wrapper cookbook, used for testing only.'

test/fixtures/cookbooks/wrapper/recipes/default.rb

@@ -0,0 +1,2 @@
+
+include 'postfix'