Remove redundant uniq

Author: fcheung

activerecord/lib/active_record/association_preload.rb

@@ -312,7 +312,7 @@ def preload_belongs_to_association(records, reflection, preload_options={})
           table_name = klass.quoted_table_name
           primary_key = klass.primary_key
           column_type = klass.columns.detect{|c| c.name == primary_key}.type
-          ids = id_map.keys.uniq.map do |id|
+          ids = id_map.keys.map do |id|
             if column_type == :integer
               id.to_i
             elsif column_type == :float