express eTag 模块是干什么的了，没读懂
这个模块主要干什么的； 以及怎么工作的 求大神解析
etag(entity, [options])
Generate a strong ETag for the given entity. This should be the complete body of the entity. Strings, Buffers, and fs.Stats are accepted. By default, a strong ETag is generated except for fs.Stats, which will generate a weak ETag (this can be overwritten by options.weak).
res.setHeader(‘ETag’, etag(body))
