def normalize_key_and_value(k, v)
      k = k.strip.downcase.gsub(/\s+/, '_').to_sym
      v = v.strip

      # check synonyms
      return k, true if %w[yes true].include?(v.downcase)
      return k, false if %w[no false].include?(v.downcase)
      return k, v
    end
def normalize_key_and_value(k, v)
      k = k.strip.downcase.gsub(/\s+/, '_').to_sym
      v = v.strip

      # check synonyms
      return k, true if %w[yes true].include?(v.downcase)
      return k, false if %w[no false].include?(v.downcase)
      return k, v
    end
Normalize the key/value pairs for email headers.
 Keys are downcased and converted to symbols;
 spaces become underscores.

 Values of `"yes"`, `"true"`, `"no"`, and `"false"`
 are converted to appropriate booleans.

 @param k [String]
 @param v [String]
 @return [Array(String, String or Boolean)]