Hash#deep_*_keys(!) recurse into nested arrays.

Following methods now recursively transform nested arrays, too.

* Hash#deep_transform_keys
* Hash#deep_transform_keys!
* Hash#deep_stringify_keys
* Hash#deep_stringify_keys!
* Hash#deep_symbolize_keys
* Hash#deep_symbolize_keys!
Merge pull request #10887 from sakuro/deep_transform_keys_in_nested_arrays

Hash#deep_*_keys(!) recurse into nested arrays.

Conflicts:
	activesupport/CHANGELOG.md