BUG: Fix for arrangedContentDidChange calling fetch()

 BUG: Fix for arrangedContentDidChange calling fetch() when _suspendedRelationships is true.

This can happen if:
 1) you have a ArrayController that relies on arrangedContent
 2) The arrangedContent points at a hasMany relationship 
 3) You call model.rollback()

Currently, arrangedContentDidChange on ManyArray does not check if the model is currently rolling back before firing fetch. 

This commit resolves this issue.