Enabled Identity insert for normal users

identity_insert_01.sql

create database identity_insert_test
GO
use identity_insert_test
exec sp_addlogin 'SimpleLogin', 'PasswordForSimpleLogin'
GO
exec sp_grantdbaccess 'SimpleLogin', 'SimpleLogin'
GO

if object_id('dbo.a') is not null
   drop table dbo.a
create table dbo.a (keyfield int identity, somevalue varchar(10))
GO

if object_id('p_a')is not null 
  drop procedure p_a
go
create procedure p_a
as
set identity_insert dbo.a on
insert into dbo.a (keyfield, somevalue) values (2,'nice')
set identity_insert dbo.a off

return (0)
go

grant exec on p_a to SimpleLogin
go