runtime: DbDataReader.Close() doesn't unify with System.Data.Common

If I write a provider against System.Data.Common on .NET Standard 1.2 and implement Dispose appropriately, then I try to use that provider on .NET Framework 4.6 or .NET Core 2.0, the following code won’t work.

reader.Close(); // No-op
Assert.True(reader.IsClosed); // Fails since Dispose is never called

It fails because on .NET Framework and .NET Core 2.0, Dispose calls Close (not the other way around) and since there is no virtual Close method on .NET Standard 1.2, I cannot override it without cross-targeting the frameworks that do have it.