Perfomance fix for Enumerable#index_by

Calculating -------------------------------------
              before    34.731k i/100ms
               after    48.206k i/100ms
-------------------------------------------------
              before    508.451k (± 1.2%) i/s -      2.570M
               after    720.068k (± 0.9%) i/s -      3.615M
Comparison:
               after:   720067.6 i/s
              before:   508451.1 i/s - 1.42x slower

activesupport/lib/active_support/core_ext/enumerable.rb

@@ -34,7 +34,9 @@ module Enumerable
   #     => { "Chade- Fowlersburg-e" => <Person ...>, "David Heinemeier Hansson" => <Person ...>, ...}
   def index_by
     if block_given?
-      Hash[map { |elem| [yield(elem), elem] }]
+      result = {}
+      each { |elem| result[yield(elem)] = elem }
+      result
     else
       to_enum(:index_by) { size if respond_to?(:size) }
     end