Fix for ListView Crash with Segmentaion Fault 11, Release _headerViewProxy observer

I noticed that _headerViewProxy observer never gets set back to nil that causes crashing my app with segmentation fault 11

objc_msgSend + 6
-[TiViewProxy relayout] (TiViewProxy.m:2243)
-[TiViewProxy refreshView:] (TiViewProxy.m:2074)
-[TiViewProxy layoutChildrenIfNeeded] (TiViewProxy.m:2274)
+[TiLayoutQueue layoutProxy:] (TiLayoutQueue.m:79)
performLayoutRefresh (TiLayoutQueue.m:46)
....

Possibley related to http://developer.appcelerator.com/question/174242/ios-crashing-when-using-native-back-button and
https://jira.appcelerator.org/browse/TC-4136