bin/cohttp_server: redirect bare directories to their slashed counterpart

Not a requirement or even a good idea... just conformance with common
servers. `dir` and `dir/` should almost certainly be collapsed but `dir`
-> `dir/` is common and `dir/` -> `dir` isn't (but is more aesthetic).
Merge pull request #164 from dsheets/bin_cohttp_server

bin/cohttp_server: redirect bare directories to their slashed counterpart