fix[Mock]: add xhr.responseType (#1779)

src/mock/index.js

@@ -11,6 +11,7 @@ Mock.XHR.prototype.proxy_send = Mock.XHR.prototype.send
 Mock.XHR.prototype.send = function() {
   if (this.custom.xhr) {
     this.custom.xhr.withCredentials = this.withCredentials || false
+    this.custom.xhr.responseType = this.responseType
   }
   this.proxy_send(...arguments)
 }