RESTful URL 设计

  POST /users/ → 201 /users/2 {"id":2,"name":"bob"}
  PUT /users/2 {name} → 200 {"id":2,"name":"bobby"}
  PATCH /users/2 {email} → 200 {"email":"b@x.test","id":2,"name":"bobby"}
  再 PUT 一次(整体替换,email 没了)→ 200 {"id":2,"name":"bobby"}
  DELETE /users/2 → 204 ''
  再 DELETE 一次 → 204 (这个实现是幂等的:删不存在的也返回 204)
  GET /users/2 → 404

  DELETE /users/ → 405 | Allow: OPTIONS, GET, HEAD, POST
  OPTIONS /users/1 → 200 | Allow: GET, HEAD, PUT, PATCH, OPTIONS, DELETE
  HEAD /users/1 → 200 | 正文长度 0 | Content-Length 22

  GET   /users    → 308 Location: http://localhost/users/
  POST  /users    → 308 Location: http://localhost/users/
  GET   /users/   → 200
  GET   /ping     → 200
  GET   /ping/    → 404

  GET /users/me  → {"me":true}
  GET /users/ann → {"by_name":"ann"}
  GET /users/1   → {"id":1,"name":"ann"}

  GET /items/12345678-1234-5678-1234-567812345678      → 200 {"i":"12345678-1234-5678-1234-567812345678","t":"UUID"}
  GET /items/not-a-uuid                                → 404 
  GET /files/a/b/c.txt                                 → 200 {"p":"a/b/c.txt"}
  GET /users/1/posts/2                                 → 200 {"pid":2,"uid":1}
  GET /users/x/posts/2                                 → 404 

  GET /v/users/1/posts/5 → 200
  GET /v/users/2/posts/5 → 404

  POST 两次:用户数 1 → 3(多了 2 条)
  PUT /users/50 两次:第一次后 4,第二次后 4(没有再增加)