Stop prompting to discard changes (again)

Once more :) an attempt to solve the problem of Umbraco continuously prompting to discard changes after saving (issue #121). 

This time around it's tested with 3 level deep nested Archetypes composed by both built in PE's and a custom PE. 

And yes, it shouldn't really be Archetype's responsibility to handle this, but if this simple fix can do the trick, I think a bunch of editors will be happy.
 
![archetypes in archetype in archetype](https://cloud.githubusercontent.com/assets/7405322/2949456/2bb6ca48-da0f-11e3-85ad-8a17b02cffa0.png)