http2: fix subsequent end calls to not throw

Calling Http2ServerResponse.end multiple times should never
cause the code to throw an error, subsequent calls should
instead return false. Fix behaviour to match http1.

Fixes: https://github.com/nodejs/node/issues/15385m

lib/internal/http2/compat.js

@@ -458,14 +458,13 @@ class Http2ServerResponse extends Stream {
       cb = encoding;
       encoding = 'utf8';
     }
+    if (stream === undefined || stream.finished === true) {
+      return false;
+    }
     if (chunk !== null && chunk !== undefined) {
       this.write(chunk, encoding);
     }
 
-    if (stream === undefined) {
-      return;
-    }
-
     if (typeof cb === 'function') {
       stream.once('finish', cb);
     }

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

@@ -4,7 +4,7 @@
 const { mustCall, mustNotCall, hasCrypto, skip } = require('../common');
 if (!hasCrypto)
   skip('missing crypto');
-const { strictEqual } = require('assert');
+const { doesNotThrow, strictEqual } = require('assert');
 const {
   createServer,
   connect,
@@ -19,6 +19,9 @@ const {
   // but may be invoked repeatedly without throwing errors.
   const server = createServer(mustCall((request, response) => {
     strictEqual(response.closed, false);
+    response.on('finish', mustCall(() => process.nextTick(
+      mustCall(() => doesNotThrow(() => response.end('test', mustNotCall())))
+    )));
     response.end(mustCall(() => {
       server.close();
     }));