Fix Rails/TimeZone cop error on invalid string literal encoding

[viralpraxis]

In some cases (e.g. for testing purposes) we might to want to use invalid string literal -- in that case IMO we should not register an offense.

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• 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.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[koic]

That's just my two cents:

Suggested change

	date.respond_to?(:value) && (!date.value.to_s.valid_encoding? || TIMEZONE_SPECIFIER.match?(date.value.to_s))
	return false unless date.respond_to?(:value)
	!date.value.to_s.valid_encoding? || TIMEZONE_SPECIFIER.match?(date.value.to_s)