Merge pull request #1 from Current-RMS/to_sym_patch

Resolve to_sym on nil error

Author: gabetlovett

lib/ransack/context.rb

@@ -152,6 +152,7 @@ def unpolymorphize_association(str)
     end
 
     def ransackable_alias(str)
+      return nil if str.nil?
       klass._ransack_aliases.fetch(str, klass._ransack_aliases.fetch(str.to_sym, str))
     end