Support array response headers in CGI

lib/rackup/handler/cgi.rb

@@ -40,8 +40,10 @@ def self.serve(app)
       def self.send_headers(status, headers)
         $stdout.print "Status: #{status}\r\n"
         headers.each { |k, vs|
-          vs.split("\n").each { |v|
-            $stdout.print "#{k}: #{v}\r\n"
+          Array(vs).each { |value|
+            value.split("\n").each { |v|
+              $stdout.print "#{k}: #{v}\r\n"
+            }
           }
         }
         $stdout.print "\r\n"

test/spec_cgi.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+# Released under the MIT License.
+# Copyright, 2026, by Oskar Eichler.
+
+require_relative 'helper'
+require_relative '../lib/rackup/handler/cgi'
+
+describe Rackup::Handler::CGI do
+  it 'writes string and array response header values' do
+    output = StringIO.new
+    original_stdout = $stdout
+    $stdout = output
+
+    Rackup::Handler::CGI.send_headers(
+      200,
+      'vary' => "accept\norigin",
+      'set-cookie' => ['session=one', 'session=two']
+    )
+
+    output.string.must_equal(
+      "Status: 200\r\n" \
+      "vary: accept\r\n" \
+      "vary: origin\r\n" \
+      "set-cookie: session=one\r\n" \
+      "set-cookie: session=two\r\n\r\n"
+    )
+  ensure
+    $stdout = original_stdout
+  end
+end