Default host to localhost when in development mode.

Running Rack apps on `0.0.0.0` in development mode will allow malicious users on the local network (ex: a Coffee Shop or a Conference) to abuse or potentially exploit the app. Safer to default host to `localhost` when in development mode.

Also default the `:Host` option to `localhost`, when in development, in `Rack::Handler::WEBrick`, `Rack::Handler::Mongrel`, `Rack::Handler::Thin`.