test: expand http2 util test coverage for headers

Expand existing test cases to hit some additional branches of
toHeaderObject and mapToHeaders.

PR-URL: nodejs/node#15493
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

test/parallel/test-http2-cookies.js

@@ -11,7 +11,8 @@ const server = h2.createServer();
 
 const setCookie = [
   'a=b',
-  'c=d; Wed, 21 Oct 2015 07:28:00 GMT; Secure; HttpOnly'
+  'c=d; Wed, 21 Oct 2015 07:28:00 GMT; Secure; HttpOnly',
+  'e=f'
 ];
 
 // we use the lower-level API here

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

@@ -261,4 +261,10 @@ const regex =
   })(mapToHeaders({ [name]: 'abc' }));
 });
 
+common.expectsError({
+  code: 'ERR_HTTP2_INVALID_CONNECTION_HEADERS',
+  message: regex
+})(mapToHeaders({ [HTTP2_HEADER_TE]: ['abc'] }));
+
 assert(!(mapToHeaders({ te: 'trailers' }) instanceof Error));
+assert(!(mapToHeaders({ te: ['trailers'] }) instanceof Error));