Fix an error for Rails/RenderPlainText when the content type is passed as a constant

[Earlopain]

Or other nodes that don't respond to value like local variables

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• Wrote good commit messages.
• 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.
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[koic]

Can you tweak to the following?

Suggested change

	if pair_node&.value.respond_to?(:value)
	pair_node.value.value == 'text/plain'
	elsif !pair_node
	!cop_config['ContentTypeCompatibility']
	end
	if pair_node.nil?
	!cop_config['ContentTypeCompatibility']
	elsif pair_node.value.respond_to?(:value)
	pair_node.value.value == 'text/plain'
	end

[koic]

Thanks!