Make ensure => absent work again

The incremental update change made NS record updates work for non-glue NS
records, but broke `ensure => absent` because there is no semantic guarantee
that the contents of the `data` member match what's in DNS. Set math ensures
that hilarity ensues.

lib/puppet_bind/provider/nsupdate.rb

@@ -83,11 +83,11 @@ module PuppetBind
       end
 
       def rrdata_adds
-        newdata - rrdata
+        resource[:ensure] === :absent ? [] : newdata - rrdata
       end
 
       def rrdata_deletes
-        type === 'SOA' ? [] : rrdata - newdata
+        resource[:ensure] === :absent ? rrdata : (type === 'SOA' ? [] : rrdata - newdata)
       end
 
       def server