Delete unnecessary line that was necessary before I changed the code and it became no longer necessary.

activerecord/lib/active_record/relation/calculations.rb

@@ -177,7 +177,6 @@ def calculate(operation, column_name, options = {})
     #   Person.where(:confirmed => true).limit(5).pluck(:id)
     #
     def pluck(column_name)
-      column_name = column_name.to_s
       klass.connection.select_all(select(column_name).arel).map! do |attributes|
         klass.type_cast_attribute(attributes.keys.first, klass.initialize_attributes(attributes))
       end