refactor: allow any streams with .pipe method

lib/application.js

@@ -215,7 +215,7 @@ function *respond(next){
   }
 
   // Stream body
-  if (body instanceof Stream) {
+  if ('function' == typeof body.pipe) {
     if (!~body.listeners('error').indexOf(this.onerror)) body.on('error', this.onerror);
 
     if (head) {

lib/response.js

@@ -133,7 +133,7 @@ module.exports = {
     }
 
     // stream
-    if (val instanceof Stream) {
+    if ('function' == typeof val.pipe) {
       if (setType) this.type = 'bin';
       return;
     }