avoid memory leakage and circular reference

[cyfung1031]

• avoid memory leakage (release xhr after XMLHttpRequest.DONE)
• avoid circular reference
• resolve proxy open方法内部，config循环引用xhr问题咨询 #116 & resolve 代理后，比axios原来的返回多了个xhr对象，导致对返回的response做JSON.stringnify处理报错 #95 - JSON.stringify would bypass property xhr

Note: since xhr is set to null in config after xhr.readyState becomes 4, config.xhr will be null in async function inside onResponse.

onResponse: function(response, handler){
     ...
      const config = response.config;
      if(config.xhr.readyState  === XMLHttpRequest.DONE){
           setTimeout(()=>{
               console.log('config.xhr === null', config.xhr===null);
          }, 100)
      }
     ...
     handler.resolve(response);
}

[cyfung1031]

Combined Distribution Script ajaxhook.js for Testing (PR 119, 120, 121, 122)
https://cdn.jsdelivr.net/gh/cyfung1031/ajax-hook@1ebe48e08108449669290a226e52fc6fbf7ec9ef/dist/ajaxhook.js