Change set_search_key_name Configuration method to be a true setter, renamed to search_key=

Related to #81

lib/ransack/configuration.rb

@@ -35,9 +35,9 @@ module Ransack
     end
 
     # default search_key that, it can be overridden on sort_link level
-    def set_search_key_name(name)
+    def search_key=(name)
       self.options[:search_key] = name
     end
 
   end
-end
+end