Description

Recent RuboCop fails with a similar error in four places:

lib/puma/state_file.rb:63:21: C: [Correctable] Performance/StringIdentifierArgument: Use :"#{f}=" instead of "#{f}=".
      define_method "#{f}=" do |v|
                    ^^^^^^^

We've used a string parameter for define_method, but Ruby's docs show it as a symbol. Also, an interpolated symbol could be used, but it's not commonly used, and these aren't 'hot' paths.

Fixed all occurrences.

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