Default host to localhost when in development mode.

* Running Rack apps on 0.0.0.0 in development mode will allow malicous
  users on the local network (ex: Coffee Shop) to abuse or
  potentially exploit the app. Safer to default host to localhost when in
  development mode.
Rack::Handler::Mongrel: default the host to localhost in development mode.
Rack::Handler::Thin: default the host to localhost in development mode.
Rack::Handler::WEBrick: default the host to localhost in development mode.