[Fix #289] count asserts within blocks for assignments

[G-Rath]

Follow up to #290 / #289 - I realised when re-reviewing my change that assertion_method? only checks the immediate expression of assignments, which makes sense for what it's meant to be doing - so we need to still handle checking the bodies of blocks for assignments.

I've not added a changelog for this since the original PR has not been released so I figured it would still be considered a single "fix", but let me know if you'd still like a second changelog entry anyway.

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• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
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• Commit message starts with [Fix #issue-number] (if the related issue exists).
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• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[koic]

Thanks!