* CORS Reference
https://developer.mozilla.org/ko/docs/Web/HTTP/CORS
https://developer.mozilla.org/ko/docs/Web/Security/Same-origin_policy
* XMLHttpRequest.open() : https://developer.mozilla.org/ko/docs/Web/API/XMLHttpRequest/open
* HTTP request methods : https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods
* HTTP 헤더 목록 참조 : https://developer.mozilla.org/ko/docs/Web/HTTP
* XMLHttpRequest.setRequestHeader() : https://developer.mozilla.org/ko/docs/Web/API/XMLHttpRequest/setRequestHeader
* Forbidden header name : https://developer.mozilla.org/en-US/docs/Glossary/Forbidden_header_name
* Forbidden response header name : https://developer.mozilla.org/en-US/docs/Glossary/Forbidden_response_header_name
* synchronous ajax error(withCredentials) : http://www.w3.org/TR/XMLHttpRequest/#the-withcredentials-attribute
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