refactor: remove duplicate assignment in respond test (#1055)

Signed-off-by: Clark Du <clark.duxin@gmail.com>

test/application/respond.js

@@ -544,7 +544,6 @@ describe('app.respond', () => {
           ctx.length = fs.readFileSync('package.json').length;
           const stream = fs.createReadStream('package.json');
           ctx.body = stream;
-          ctx.body = stream;
           ctx.set('Content-Type', 'application/json; charset=utf-8');
         });