在node中用了一下request.head，发现很少关注过method为HEAD的：
The HEAD method is identical to the GET method except that the server must not return a message-body in the response. The metainformation contained in the HTTP headers in response to a HEAD request should be identical to the information sent in response to a GET request. This allows a client to obtain meta-information about a resource without actually transferring the resource itself.
The head method is often used for testing hyperlinks, accessibility and for determining if a document has been recently modifed.
When your program is implementing caching, it is important to note that if the HEAD response indicates that the cached entity differs from the current entity, such as by a change in the Content-Length, Content-MD5, ETag or Last-Modified, the cache must treat the cached entry as stale.
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