www_static: support default_type parameter

www_static: support default_type parameter

Sysadmins may wish to override the default
`application/octet-stream' for files with no suffix or unknown
suffixes.  One use case is serving the
https://public-inbox.org/README itself with the `text/plain'
Content-Type.

Plack::MIME->set_fallback can be an alternative to this
parameter, but its effect is interpreter-wide and server admins
may wish to set the default type on a per-route basis.