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.
all header field names in both HTTP requests and HTTP responses are case-insensitive.
原文：https://www.cnblogs.com/yuanjiangw/p/12964835.html