fix(res.sendFile): treat the app etag setting as a default, not an override

lib/response.js

@@ -398,8 +398,9 @@ res.sendFile = function sendFile(path, options, callback) {
   // create file stream
   var pathname = encodeURI(path);
 
+  opts = Object.create(opts);
   // wire application etag option to send
-  opts.etag = this.app.enabled('etag');
+  opts.etag ??= this.app.enabled('etag');
   var file = send(req, pathname, opts);
 
   // transfer

test/res.sendFile.js

@@ -92,6 +92,39 @@ describe('res', function(){
       }
     });
 
+    it('should let an explicit etag option turn the ETag off', function (done) {
+      var app = createApp(path.resolve(fixtures, 'name.txt'), { etag: false });
+
+      request(app)
+      .get('/')
+      .expect(handleHeaders)
+      .expect(200, done);
+
+      function handleHeaders (res) {
+        assert(res.headers.etag === undefined);
+      }
+    });
+
+    it('should let an explicit etag option turn the ETag on', function (done) {
+      var app = createApp(path.resolve(fixtures, 'name.txt'), { etag: true }).disable('etag');
+
+      request(app)
+      .get('/')
+      .expect('ETag', /^(?:W\/)?"[^"]+"$/)
+      .expect(200, done);
+    });
+
+    it('should not modify the options object', function (done) {
+      var options = {};
+      var app = createApp(path.resolve(fixtures, 'name.txt'), options).disable('etag');
+
+      request(app)
+      .get('/')
+      .expect(utils.shouldNotHaveHeader('ETag'))
+      .expect(() => assert.deepStrictEqual(options, {}))
+      .expect(200, done);
+    });
+
     it('should 404 for directory', function (done) {
       var app = createApp(path.resolve(fixtures, 'blog'));