Use deep_dup in the deep_transform_keys tests.
Use deep_dup in the deep_transform_keys tests.

Using only dup make some tests to not catch up an implementation error
because the methods were changing the nested hashes.

Related to:
https://github.com/rails/rails/commit/541429fbe49b0671adb3842ab1818230d670ef9f#L1R96

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