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
Make default-content-type return <byte-string>
Update locator-media-type

Change the behaviour of locator-media-type to reflect the new generic default-content-type. Remove the specialized default-content-type as well since it is no longer needed.
Update dsp.dylan

Change the specialization of default-content-type to fit with the new generic interface.
Merge pull request #59 from kibook/patch-1

default-content-type not specialized