Prevent mutation of unsaved AR collections by Marshal#dump

Marshal#dump calls CollectionProxy#respond_to? when dumping some AR
collections. The call to load_target may mutate the internal state
mid-dump, resulting in undesirable queries and a dumped string that may
not result in a usable object when processed by Marshal#load.
Merge pull request #1 from mirakui/3-2-regression-fixed

Prevent mutation of unsaved AR collections by Marshal#dump