The http spec says about the HEAD request:
The HEAD method is identical to GET 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.
Should the response to a HEAD request contain a Content-Length header? Should it be the value which would be returned on a GET request, even if there is no response body? Or should the Content-Length be 0?
To me it looks like the HTTP 1.1 RFC is pretty specific:
The Content-Length entity-header field indicates the size of the entity-body, in decimal number of OCTETs, sent to the recipient or, in the case of the HEAD method, the size of the entity-body that would have been sent had the request been a GET.