Add a spec for Enumerable#chunk when given a mutable initial_state
Fix Enumerable#chunk when given a mutable initial_state

* Duplicate only the every first iteration. Not every iterations.
* Use #dup instead of #clone to duplicate the object.
Merge pull request #2440 from kachick/fix-enum-chunk-with_init

Fix Enumerable#chunk when given a mutable initial_state