Proposal: extend the fetch call to be used to only fetch HEAD

See original blocked enhancement request: https://github.com/npm/npm/pull/6208
Since the 'add-remote-tarball' module invokes the 'registry.fetch' command (that require the correct application/x-tar MIME type), I've tried to modify the fetch call itself to support HEAD method.

However I'm not sure what other users depends on that call, so I'm proposing an ugly workaround to pass in the method arg without changing the args (I've seen that the callback 'cb' par is always at the end of the list, so adding a new parameter seems more ugly).

Thank you,
 Luciano