Manually add Host header when passed as Array

Notably, when `request#options.headers` is an Array, the `Host`
header won't be automatically set. In prior versions of node
this wasn't of much consequence, but as of [1] its enforced.

[1] nodejs/node#45597

test/helpers/index.js

@@ -39,7 +39,12 @@ export const req = ({
     }
 
     if (Array.isArray(headers)) {
-      headers = [...Object.entries(bodyHeaders).flat(), ...headers];
+      headers = [
+        ...Object.entries(bodyHeaders).flat(),
+        'Host',
+        `localhost:${port}`,
+        ...headers,
+      ];
     } else {
       headers = { ...bodyHeaders, ...headers };
     }