remove depreciated assertion to eliminate warning

[xtina-starr]

This PR removes the following warning by using assert_nil over assert_equal which is depreciated.

Use assert_nil if expecting nil from .../rails/activesupport/test/hash_with_indifferent_access_test.rb:563:in `test_double_conversion_with_nil_key'. This will fail in MT6.

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[rafaelfranca]

Can you seach all the places that does that and remove they in one single commit?

[xtina-starr]

$ ag "assert_equal nil" .
activesupport/test/hash_with_indifferent_access_test.rb
563:    assert_equal nil, h[:undefined_key]

After executing the above it looks like that's the only place assert_equal is used.