all header field names in both HTTP requests and HTTP responses are case-insensitive.

https://tools.ietf.org/html/rfc6455#section-4.2.1

 Please note that according to [RFC2616], all header field names in
   both HTTP requests and HTTP responses are case-insensitive.

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原文地址：https://www.cnblogs.com/rsapaper/p/12964835.html