[BUGFIX] Fix for ManyArray.arrangedContentDidChange calling fetch()

[BUGFIX] Fix for ManyArray.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 (the ManyArray)
 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.