Directs all traffic to a single domain via 301 redirects.
use Rack::ForceDomain, ENV["DOMAIN"]
config.middleware.use Rack::ForceDomain, ENV["DOMAIN"]
heroku config:add DOMAIN="foo.com"
Now all requests to www.foo.com (or anything else pointed at the app) will 301 to foo.com.
$DOMAIN environment variable is missing, no redirection will occur.
You can also give provide a port along with your domain "foo.com:3000".