Use Hash#each_key instead of Hash#keys.each

Hash#keys.each allocates an array of keys.

Hash#each_key iterates through the keys without allocating a
new array and as such is a better, more specific tool for the
job.

lib/ransack/search.rb

@@ -152,7 +152,7 @@ module Ransack
     end
 
     def collapse_multiparameter_attributes!(attrs)
-      attrs.keys.each do |k|
+      attrs.each_key do |k|
         if k.include?(Constants::LEFT_PARENTHESIS)
           real_attribute, position = k.split(/\(|\)/)
           cast =