http2: simplify te header validation, add test

This simplifies validation of te header and adds a test case
in which array with two values is passed to param value in
isIllegalConnectionSpecificHeader

PR-URL: nodejs/node#16246
Refs: nodejs/node#14985
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>

lib/internal/http2/util.js

@@ -360,8 +360,7 @@ function isIllegalConnectionSpecificHeader(name, value) {
     case HTTP2_HEADER_TRANSFER_ENCODING:
       return true;
     case HTTP2_HEADER_TE:
-      const val = Array.isArray(value) ? value.join(', ') : value;
-      return val !== 'trailers';
+      return value !== 'trailers';
     default:
       return false;
   }

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

@@ -266,5 +266,10 @@ common.expectsError({
   message: regex
 })(mapToHeaders({ [HTTP2_HEADER_TE]: ['abc'] }));
 
+common.expectsError({
+  code: 'ERR_HTTP2_INVALID_CONNECTION_HEADERS',
+  message: regex
+})(mapToHeaders({ [HTTP2_HEADER_TE]: ['abc', 'trailers'] }));
+
 assert(!(mapToHeaders({ te: 'trailers' }) instanceof Error));
 assert(!(mapToHeaders({ te: ['trailers'] }) instanceof Error));