fix length when .body is missing

lib/application.js

@@ -190,6 +190,8 @@ function *respond(next) {
     // status body
     this.type = 'text';
     body = status[code];
+    if (body) this.length = Buffer.byteLength(body);
+    return res.end(body);
   }
 
   // Buffer body

test/application.js

@@ -288,6 +288,7 @@ describe('app.respond', function(){
         request(server)
         .get('/')
         .expect(400)
+        .expect('Content-Length', 11)
         .expect('Bad Request', done);
       })
     })