HashWithIndifferentAccess#update respects #to_hash

Hash#update converts its argument with #to_hash before looking at its values.
This change makes HashWithIndifferentAccess do the same thing.

I'd like to write a test to go with this, but I couldn't find a test for
HashWithIndifferentAccess. Is there one?

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* [x] Test (in `test/core_ext/has_ext_test.rb`)
* [x] Changelog entry