perf: avoid stringify when set header (#1220)

lib/response.js

@@ -439,8 +439,8 @@ module.exports = {
     if (this.headerSent) return;
 
     if (2 == arguments.length) {
-      if (Array.isArray(val)) val = val.map(String);
-      else val = String(val);
+      if (Array.isArray(val)) val = val.map(v => typeof v === 'string' ? v : String(v));
+      else if (typeof val !== 'string') val = String(val);
       this.res.setHeader(field, val);
     } else {
       for (const key in field) {

test/response/header.js

@@ -10,7 +10,8 @@ describe('res.header', () => {
   it('should return the response header object', () => {
     const res = response();
     res.set('X-Foo', 'bar');
-    assert.deepEqual(res.header, { 'x-foo': 'bar' });
+    res.set('X-Number', 200);
+    assert.deepEqual(res.header, { 'x-foo': 'bar', 'x-number': '200' });
   });
 
   it('should use res.getHeaders() accessor when available', () => {

test/response/set.js

@@ -11,12 +11,18 @@ describe('ctx.set(name, val)', () => {
     assert.equal(ctx.response.header['x-foo'], 'bar');
   });
 
-  it('should coerce to a string', () => {
+  it('should coerce number to string', () => {
     const ctx = context();
     ctx.set('x-foo', 5);
     assert.equal(ctx.response.header['x-foo'], '5');
   });
 
+  it('should coerce undefined to string', () => {
+    const ctx = context();
+    ctx.set('x-foo', undefined);
+    assert.equal(ctx.response.header['x-foo'], 'undefined');
+  });
+
   it('should set a field value of array', () => {
     const ctx = context();
     ctx.set('x-foo', ['foo', 'bar']);