DbContext: ExecuteSqlCommand has poor usability when used with params and cancellation tokens

[anpete]

1. When passing a strongly typed array for the parameters arg: E.g.

context.Database.ExecuteSqlCommand(
    "SELECT * FROM Orders WHERE OrderID = {0} OR CustomerID = {1}", 
    new [] { 1, 2 }));

We crash because the parameters object[] ends up with a single int[] element.

2. The Async overloads require passing a cancellation token when specifying parameters.

Test coverage of these methods is low. We should beef it up.

[rowanmiller]

We should look at adopting the pattern we use elsewhere for async methods (not combining params and CancellationToken). We should also look at an overload that takes an IEnumerable of parameters rather than params.