4.1. 304 Not Modified
返回304时如何携带Header
必须携带的Header+可以携带的用于指导缓存的Header
必须(MUST)携带的Header
// MUST
The server generating a 304 response MUST generate any of the
following header fields that would have been sent in a 200 (OK)
response to the same request: Cache-Control, Content-Location, Date,
ETag, Expires, and Vary.
- 上述列表中的header fields，如果在给request返回200时，返回了，则当给这个request返回304时也要(MUST)返回。
可以携带用于指导缓存更新的Header
// SHOULD NOT
Since the goal of a 304 response is to minimize information transfer
when the recipient already has one or more cached representations, a
sender SHOULD NOT generate representation metadata other than the
above listed fields unless said metadata exists for the purpose of
guiding cache updates (e.g., Last-Modified might be useful if the
response does not have an ETag field).
上述节选定义了返回304时，如何返回metadata：
- sender不应该(SHOULD NOT)发送上述列表以外的metadata，除非它能帮助提升缓存更新（例如发送Last-Modified当ETag生成失败时）
Requirements on a cache that receives a 304 response are defined in
Section 4.3.4 of [RFC7234]. If the conditional request originated
with an outbound client, such as a user agent with its own cache
sending a conditional GET to a shared proxy, then the proxy SHOULD
forward the 304 response to that client.
返回304时如何携带body
304 response不能携带body。gemini说这里的"cannot"语义等同于"MUST NOT"
