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: #16452
PR-URL: #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>

Author: jinwoo

lib/internal/http2/util.js

@@ -403,13 +403,14 @@ function mapToHeaders(map,
 if(typeofkey==='symbol'||value===undefined||!key)
 continue;
 key=String(key).toLowerCase();
-constisArray=Array.isArray(value);
+letisArray=Array.isArray(value);
 if(isArray){
 switch(value.length){
 case0:
 continue;
 case1:
 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)
+constheaders={
+'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,