Incorrect migration for changing the key from manual ID to IDENTITY

[mafshin]

Steps to reproduce

Having this model

    public class Book
    {
        public int ID { get; set; }
        public string Name { get; set; }
    }

    public class BookContext : DbContext
    {
        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            optionsBuilder.UseSqlServer("Data Source=.\\SQLExpress;Initial Catalog=Books;Integrated Security=True");

            base.OnConfiguring(optionsBuilder);
        }
        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Book>().Property(x => x.ID).ValueGeneratedNever();            

            base.OnModelCreating(modelBuilder);
        }

        public DbSet<Book> Books { get; set; }
    }

in which Book.ID must be set manually:

• Add-Migration Initial
• Update-Database
• Make the ID property to be IDENTITY by commenting ValueGeneratedNever() line

            //modelBuilder.Entity<Book>().Property(x => x.ID).ValueGeneratedNever();   

• Add-Migration Set_Book_ID_As_Identity
• Update-Database

The issue

The generated migration for setting the ID to IDENTITY is as follows:

  public partial class Set_Book_ID_As_Identity : Migration
    {
        protected override void Up(MigrationBuilder migrationBuilder)
        {
            migrationBuilder.AlterColumn<int>(
                name: "ID",
                table: "Books",
                nullable: false)
                .Annotation("SqlServer:ValueGenerationStrategy", SqlServerValueGenerationStrategy.IdentityColumn);
        }

        protected override void Down(MigrationBuilder migrationBuilder)
        {
            migrationBuilder.AlterColumn<int>(
                name: "ID",
                table: "Books",
                nullable: false);
        }
    }

resulting in this SQL

ALTER TABLE [Books] ALTER COLUMN [ID] int NOT NULL;

which doesn't set the ID to auto-increment identity.

Expected behavior

To generate a migration like this

            migrationBuilder.DropColumn(
                 name: "ID",
                 table: "Books");

             migrationBuilder.AddColumn<int>(
                name: "ID",
                table: "Books",
                nullable: false)
                .Annotation("SqlServer:ValueGenerationStrategy", SqlServerValueGenerationStrategy.IdentityColumn);

Further technical details

EF Core version: 1.0
Operating system: Windows 10 x64
Visual Studio version: VS 2015 Update 3

[bricelam]

Dupe of #2100 (tracked as part of #329)