Fix for "undefined method encoding' for nil:NilClass"

This code: ```ruby img src="/xyz/#{@house.address}" ``` Caused this: ```ruby undefined method encoding' for nil:NilClass /usr/local/rbenv/versions/2.0.0-p247/lib/ruby/2.0.0/cgi/util.rb in escape encoding = string.encoding ``` Why? Because address is nil and Slim tried to escape nil with CGI.escape, which doesn't work. The error was reported to be on line x when the error was on line y…