Add a trait for managing download responses

After doing a bit of searching, I didn't find anything that would help
with sending a downloaded response, instead of the default, which is to
send a text response. So I created this one, which sends six headers
that ensure that the response sent will be interpreted by the client as
a downloadable file.

Author: settermjd

src/Response/DownloadResponseTrait.php

@@ -0,0 +1,83 @@
+<?php
+/**
+ * @see       https://github.com/zendframework/zend-diactoros for the canonical source repository
+ * @copyright Copyright (c) 2015-2018 Zend Technologies USA Inc. (https://www.zend.com)
+ * @license   https://github.com/zendframework/zend-diactoros/blob/master/LICENSE.md New BSD License
+ */
+
+declare(strict_types=1);
+
+namespace Zend\Diactoros\Response;
+
+use Zend\Diactoros\Exception\InvalidArgumentException;
+
+use function array_keys;
+use function array_merge;
+use function implode;
+use function in_array;
+use function sprintf;
+
+/**
+ * Trait DownloadResponseTrait
+ * @package Zend\Diactoros\Response
+ */
+trait DownloadResponseTrait
+{
+    /**
+     * A list of header keys required to be sent with a download response
+     *
+     * @var array
+     */
+    private $downloadResponseHeaders = [
+        'cache-control',
+        'content-description',
+        'content-disposition',
+        'content-transfer-encoding',
+        'expires',
+        'pragma'
+    ];
+
+    /**
+     * Check if the extra headers contain any of the download headers
+     *
+     * The download headers cannot be overridden.
+     *
+     * @param array $downloadHeaders
+     * @param array $headers
+     * @return bool
+     */
+    public function overridesDownloadHeaders(array $downloadHeaders, array $headers = []) : bool
+    {
+        $overridesDownloadHeaders = false;
+
+        foreach (array_keys($headers) as $header) {
+            if (in_array($header, $downloadHeaders)) {
+                $overridesDownloadHeaders = true;
+                break;
+            }
+        }
+
+        return $overridesDownloadHeaders;
+    }
+
+    /**
+     * Prepare download response headers
+     *
+     * @param string $filename
+     * @param array $headers
+     * @return array
+     */
+    private function prepareDownloadHeaders(string $filename, array $headers = []) : array
+    {
+        if ($this->overridesDownloadHeaders($this->downloadResponseHeaders, $headers)) {
+            throw new InvalidArgumentException(
+                sprintf(
+                    'Cannot override download headers (%s) when download response is being sent',
+                    implode(', ', $this->downloadResponseHeaders)
+                )
+            );
+        }
+
+        return array_merge($headers, $this->getDownloadHeaders($filename));
+    }
+}