http2: do no throw in writeHead if state.closed

The http1 implementation does not throw if the connection is down.
The http2 compat implementation should do the same.

See: fastify/fastify-http-proxy#51.
See: fastify/fastify#1494.

lib/internal/http2/compat.js

@@ -589,14 +589,11 @@ class Http2ServerResponse extends Stream {
   writeHead(statusCode, statusMessage, headers) {
     const state = this[kState];
 
-    if (state.closed)
-      throw new ERR_HTTP2_INVALID_STREAM();
+    if (state.closed || this.stream.destroyed)
+      return this;
     if (this[kStream].headersSent)
       throw new ERR_HTTP2_HEADERS_SENT();
 
-    if (this.stream.destroyed)
-      return this;
-
     if (typeof statusMessage === 'string')
       statusMessageWarn();
 

test/parallel/test-http2-compat-serverresponse-writehead.js

@@ -26,9 +26,9 @@ server.listen(0, common.mustCall(function() {
     response.on('finish', common.mustCall(function() {
       server.close();
       process.nextTick(common.mustCall(() => {
-        common.expectsError(() => { response.writeHead(300); }, {
-          code: 'ERR_HTTP2_INVALID_STREAM'
-        });
+        // The stream is invalid at this point,
+        // and this line verifies this does not throw.
+        response.writeHead(300);
       }));
     }));
     response.end();