use the database to lower case strings rather than ruby. [#4293 state:resolved]

Signed-off-by: Nwycats <wycats@gmail.com>

activerecord/lib/active_record/validations/uniqueness.rb

@@ -67,13 +67,11 @@ def mount_sql_and_params(klass, table_name, attribute, value) #:nodoc:
 
         if value.nil? || (options[:case_sensitive] || !column.text?)
           sql = "#{sql_attribute} #{operator}"
-          params = [value]
         else
-          sql = "LOWER(#{sql_attribute}) #{operator}"
-          params = [value.mb_chars.downcase]
+          sql = "LOWER(#{sql_attribute}) = LOWER(?)"
         end
 
-        [sql, params]
+        [sql, [value]]
       end
     end