Add the 'Last-Modified' header to the default file handler

The default file handler is missing the`Last-Modified` header, this doesn't allow the browser to cache any resource.

Found this problem with a mozmill/mutt test that was checking "http-on-examine-response" vs "http-on-examine-cached-response" observers, and in the case of wptserve we never got any resource from the local cache.

This PR fixes the problem by setting this header.

@jgraham please review when you have time. Thanks