认证挑战表明对于客户端来说什么机制在未来的请求中提供认证凭证时可用的。

   +--------------------+--------------------------+
   | Header Field Name  | Defined in...            |
   +--------------------+--------------------------+
   | WWW-Authenticate   | Section 4.1 of [RFC7235] |
   | Proxy-Authenticate | Section 4.3 of [RFC7235] |
   +--------------------+--------------------------+