to_param when we have no slug

Remember when we needed to ask in the places view if ```slug.nil?```, to avoid the scary exception? 
Today we learn how to deal with this more gracefully (so it's not needed in the view), and more important "repair" the place data directly in case of an error. 

some resources, in case you get stuck: 
- http://www.mikeperham.com/2012/03/03/the-perils-of-rescue-exception/
- http://www.rubyinside.com/21-ruby-tricks-902.html

Have fun!