SaveChangesAsync() doc page

Moved from https://github.com/aspnet/EntityFrameworkCore/issues/12751 posted by @erictrigo

Check out: https://docs.microsoft.com/en-us/dotnet/api/microsoft.entityframeworkcore.dbcontext.savechangesasync?view=efcore-2.1

Says signature for SaveChangesAsync is

`public virtual System.Threading.Tasks.Task SaveChangesAsync (System.Threading.CancellationToken cancellationToken = null);`

If you try overriding SaveChangesAsync using the above example, it'll give you the following error:


Using `default(CancellationToken)` instead of `null` solves the issue:
`public override async Task SaveChangesAsync(CancellationToken cancellationToken = default(CancellationToken))`