test: increase coverage of _http_outgoing

validateHeader: https://github.com/nodejs/node/blob/master/lib/_http_outgoing.js#L376
write: https://github.com/nodejs/node/blob/master/lib/_http_outgoing.js#L477
addTrailers: https://github.com/nodejs/node/blob/master/lib/_http_outgoing.js#L557

PR-URL: #10820
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

test/parallel/test-http-outgoing-proto.js

@@ -12,3 +12,63 @@ assert.strictEqual(
   typeof ClientRequest.prototype._implicitHeader, 'function');
 assert.strictEqual(
   typeof ServerResponse.prototype._implicitHeader, 'function');
+
+// validateHeader
+assert.throws(() => {
+  const outgoingMessage = new OutgoingMessage();
+  outgoingMessage.setHeader();
+}, /^TypeError: Header name must be a valid HTTP Token \["undefined"\]$/);
+
+assert.throws(() => {
+  const outgoingMessage = new OutgoingMessage();
+  outgoingMessage.setHeader('test');
+}, /^Error: "value" required in setHeader\("test", value\)$/);
+
+assert.throws(() => {
+  const outgoingMessage = new OutgoingMessage();
+  outgoingMessage.setHeader(404);
+}, /^TypeError: Header name must be a valid HTTP Token \["404"\]$/);
+
+assert.throws(() => {
+  const outgoingMessage = new OutgoingMessage();
+  outgoingMessage.setHeader.call({_header: 'test'}, 'test', 'value');
+}, /^Error: Can't set headers after they are sent.$/);
+
+assert.throws(() => {
+  const outgoingMessage = new OutgoingMessage();
+  outgoingMessage.setHeader('200', 'あ');
+}, /^TypeError: The header content contains invalid characters$/);
+
+// write
+assert.throws(() => {
+  const outgoingMessage = new OutgoingMessage();
+  outgoingMessage.write();
+}, /^Error: _implicitHeader\(\) method is not implemented$/);
+
+assert(OutgoingMessage.prototype.write.call({_header: 'test'}));
+
+assert.throws(() => {
+  const outgoingMessage = new OutgoingMessage();
+  outgoingMessage.write.call({_header: 'test', _hasBody: 'test'});
+}, /^TypeError: First argument must be a string or Buffer$/);
+
+assert.throws(() => {
+  const outgoingMessage = new OutgoingMessage();
+  outgoingMessage.write.call({_header: 'test', _hasBody: 'test'}, 1);
+}, /^TypeError: First argument must be a string or Buffer$/);
+
+// addTrailers
+assert.throws(() => {
+  const outgoingMessage = new OutgoingMessage();
+  outgoingMessage.addTrailers();
+}, /^TypeError: Cannot convert undefined or null to object$/);
+
+assert.throws(() => {
+  const outgoingMessage = new OutgoingMessage();
+  outgoingMessage.addTrailers({'あ': 'value'});
+}, /^TypeError: Trailer name must be a valid HTTP Token \["あ"\]$/);
+
+assert.throws(() => {
+  const outgoingMessage = new OutgoingMessage();
+  outgoingMessage.addTrailers({404: 'あ'});
+}, /^TypeError: The trailer content contains invalid characters$/);