http: unify header treatment

lib/_http_outgoing.js

@@ -359,7 +359,7 @@ OutgoingMessage.prototype._send = function _send(data, encoding, callback) {
     // `this._header` can be null if OutgoingMessage is used without a proper Socket
     // See: /test/parallel/test-http-outgoing-message-inheritance.js
     if (typeof data === 'string' &&
-        (encoding === 'utf8' || encoding === 'latin1' || !encoding)) {
+        (encoding === 'utf8' || encoding === 'latin1')) {
       data = this._header + data;
     } else {
       const header = this._header;

test/parallel/test-http-server-non-utf8-header.js

@@ -0,0 +1,48 @@
+'use strict';
+const common = require('../common');
+const http = require('http');
+const assert = require('assert');
+
+const nonUtf8Header = 'bår';
+const nonUtf8ToLatin1 = Buffer.from(nonUtf8Header).toString('latin1');
+
+{
+  const server = http.createServer(common.mustCall((req, res) => {
+    res.writeHead(200, [
+      'foo',
+      Buffer.from(nonUtf8Header).toString('binary'),
+    ]);
+    res.end('hello');
+  }));
+
+  server.listen(0, common.mustCall(() => {
+    http.get({ port: server.address().port }, (res) => {
+      assert.strictEqual(res.statusCode, 200);
+      assert.strictEqual(res.headers.foo, nonUtf8ToLatin1);
+      res.resume().on('end', common.mustCall(() => {
+        server.close();
+      }));
+    });
+  }));
+}
+
+{
+  const server = http.createServer(common.mustCall((req, res) => {
+    res.writeHead(200, [
+      'Content-Length', '5',
+      'foo',
+      Buffer.from(nonUtf8Header).toString('binary'),
+    ]);
+    res.end('hello');
+  }));
+
+  server.listen(0, common.mustCall(() => {
+    http.get({ port: server.address().port }, (res) => {
+      assert.strictEqual(res.statusCode, 200);
+      assert.strictEqual(res.headers.foo, nonUtf8ToLatin1);
+      res.resume().on('end', common.mustCall(() => {
+        server.close();
+      }));
+    });
+  }));
+}

test/parallel/test-http-server-response-standalone.js

@@ -20,13 +20,11 @@ let firstChunk = true;
 const ws = new Writable({
   write: common.mustCall((chunk, encoding, callback) => {
     if (firstChunk) {
-      assert(chunk.toString().endsWith('hello world'));
+      assert(chunk.toString().startsWith('HTTP/1.1 200 OK'));
       firstChunk = false;
-    } else {
-      assert.strictEqual(chunk.length, 0);
     }
     setImmediate(callback);
-  }, 2)
+  }, 3)
 });
 
 res.assignSocket(ws);