HTTP Header 是不区分大小写的

Are HTTP headers case-sensitive? - Stack Overflow

那为什么 Mozilla 在示例里还要将头的值都小写处理呢？

var targetPage = "http://useragentstring.com/*";

var ua = "Opera/9.80 (X11; Linux i686; Ubuntu/14.10) Presto/2.12.388 Version/12.16";

function rewriteUserAgentHeader(e) {
  for (var header of e.requestHeaders) {
    if (header.name.toLowerCase() == "user-agent") {
      header.value = ua;
    }
  }
  return {requestHeaders: e.requestHeaders};
}

chrome.webRequest.onBeforeSendHeaders.addListener(
  rewriteUserAgentHeader,
  {urls: [targetPage]},
  ["blocking", "requestHeaders"]
);