fix const_get vs rails issue

Author: grosser

lib/zendesk_api/association.rb

@@ -231,7 +231,7 @@ def get_class(resource)
 
  begin
  const_get(res)
- rescue NameError
+ rescue NameError, ArgumentError # ruby raises NameError, rails raises ArgumentError
  ZendeskAPI.get_class(resource)
  end
  end