default-content-type not specialized

f83e9d44a4cd1bceb88a1fc7c6a1b28488a953a9 allows default-content-type to return either a <mime-type> or a <string> (and specializes to <string> with "application/octet-stream"), but locator-media-type for <directory-resource> here expects only a <mime-type>.

Add a specialized default-content-type method for <directory-resource> which returns a new <mime-type> of application/octet-stream