afa99ad294e7+ tip

From RFC 2616:

9.4 HEAD

   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. This method can
   be used for obtaining metainformation about the entity implied by the
   request without transferring the entity-body itself. This method is
   often used for testing hypertext links for validity, accessibility,
   and recent modification.

14.17 Content-Type

   The Content-Type entity-header field indicates the media type of the
   entity-body sent to the recipient or, in the case of the HEAD method,
   the media type that would have been sent had the request been a GET.

Since it SHOULD be identical to the GET request, it is not mandatory--and it seems
unnecessary to read the body to infer the content-type just for HEAD requests--but we
should not be returning an incorrect Content-Type (text/plain) for HEAD requests for
e.g. mp3, html files.