[]=

# File actionmailer/lib/action_mailer/base.rb, line 479
      def []=(key, value)
        unless value.is_a?(::String)
          ::ActiveSupport::Deprecation.warn("Using a non-String object for a header's value is deprecated. "              "You specified #{value.inspect} (a #{value.class}) for #{key}", caller)

          value = value.to_s
        end

        @message[key] = value
      end