unset content-type when the type is unknown

closes #532
closes #536

lib/response.js

@@ -301,7 +301,12 @@ module.exports = {
    */
 
   set type(type) {
-    this.set('Content-Type', getType(type) || 'application/octet-stream');
+    type = getType(type) || false;
+    if (type) {
+      this.set('Content-Type', type);
+    } else {
+      this.remove('Content-Type');
+    }
   },
 
   /**

test/application/respond.js

@@ -34,6 +34,28 @@ describe('app.respond', function(){
     });
   });
 
+  describe('when this.type === null', function(){
+    it('should not send Content-Type header', function(done){
+      var app = new Koa();
+
+      app.use(function *(){
+        this.body = '';
+        this.type = null;
+      });
+
+      var server = app.listen();
+
+      request(server)
+        .get('/')
+        .expect(200)
+        .end(function(err, res){
+          if (err) return done(err);
+          res.should.not.have.header('content-type');
+          done();
+        });
+    });
+  });
+
   describe('when HEAD is used', function(){
     it('should not respond with the body', function(done){
       const app = new Koa();

test/response/type.js

@@ -42,11 +42,11 @@ describe('ctx.type=', function(){
   });
 
   describe('with an unknown extension', function(){
-    it('should default to application/octet-stream', function(){
+    it('should not set a content-type', function(){
-      const ctx = context();
+      var ctx = context();
       ctx.type = 'asdf';
-      ctx.type.should.equal('application/octet-stream');
+      assert(!ctx.type);
-      ctx.response.header['content-type'].should.equal('application/octet-stream');
+      assert(!ctx.response.header['content-type']);
     });
   });
 });