使用AngularJS将字符串数据发布到Web API会导致null

I'm using AngularJS v1.2.25. When I post a payload data (string) to my Web API via POST I always get null. I tried it with Postman (REST client), then I get a string. I was wondering whether this is a bug, because the typeof variable is string.I'm using AngularJS v1.2.25. When I post a payl