The XMLHttpRequest method setRequestHeader() sets the value of an HTTP request header. When using setRequestHeader(), you must call it after calling open(), but before calling send(). If this method is called several times with the same header, the values are merged into one single request header.
Each time you call setRequestHeader() after the first time you call it, the specified text is appended to the end of the existing header's content.
If no Accept header has been set using this, an Accept header with the type "*/*" is sent with the request when send() is called.
For security reasons, some headers can only be controlled by the user agent. These headers include the forbidden header names and forbidden response header names.
Note: For your custom fields, you may encounter a "not allowed by Access-Control-Allow-Headers in preflight response" exception when you send requests across domains. In this situation, you need to set up the Access-Control-Allow-Headers in your response header at server side.
