http2: fix mapToHeaders() with single string value

This is for issue 16452. When 'set-cookie' header is set with an array
that has only one string value, it's split into its individual
characters.

Fix by resetting `isArray` to false when the value is converted from an
array to a string.

Fixes: https://github.com/nodejs/node/issues/16452
PR-URL: https://github.com/nodejs/node/pull/16458
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>

lib/internal/http2/util.js

@@ -403,13 +403,14 @@ function mapToHeaders(map,
     if (typeof key === 'symbol' || value === undefined || !key)
       continue;
     key = String(key).toLowerCase();
-    const isArray = Array.isArray(value);
+    let isArray = Array.isArray(value);
     if (isArray) {
       switch (value.length) {
         case 0:
           continue;
         case 1:
           value = String(value[0]);
+          isArray = false;
           break;
         default:
           if (kSingleValueHeaders.has(key))

test/parallel/test-http2-util-headers-list.js

@@ -154,6 +154,18 @@ const {
   );
 }
 
+{
+  // Arrays containing a single set-cookie value are handled correctly
+  // (https://github.com/nodejs/node/issues/16452)
+  const headers = {
+    'set-cookie': 'foo=bar'
+  };
+  assert.deepStrictEqual(
+    mapToHeaders(headers),
+    [ [ 'set-cookie', 'foo=bar', '' ].join('\0'), 1 ]
+  );
+}
+
 // The following are not allowed to have multiple values
 [
   HTTP2_HEADER_STATUS,