[BUG] Invalidate not working properly for InMemory

[thisisthekap]

Describe the bug

Invalidate seems not to do proper invalidation, as the subsequent call to GetOrFetchObject returns the previously gathered value. See the example below for details.

Steps To Reproduce

class Program
{
    private static int _cnt;

    static async Task Main(string[] args)
    {
        BlobCache.ApplicationName = $"akavache-{Guid.NewGuid()}";

        await TestIt(BlobCache.LocalMachine);
        await TestIt(BlobCache.InMemory);
    }

    private static async Task TestIt(IBlobCache cache)
    {
        _cnt = 0;
        Console.WriteLine(await GetOrFetchAsync(cache, DateTime.UtcNow + TimeSpan.FromMilliseconds(1000)));
        await cache.Invalidate("a");
        Console.WriteLine(await GetOrFetchAsync(cache, DateTime.UtcNow + TimeSpan.FromMilliseconds(1000)));
        Console.WriteLine("=====================");
        Console.WriteLine();
    }

    private static async Task<string> GetOrFetchAsync(IBlobCache cache, DateTimeOffset? absoluteExpiration)
    {
        return await cache.GetOrFetchObject("a", async () =>
        {
            await Task.Delay(500);
            var a = $"b{Interlocked.Increment(ref _cnt)}";
            Console.WriteLine("A:" + a);
            return a;
        }, absoluteExpiration);
    }
}

Expected behavior

A:b1
b1
A:b2
b2
=====================

A:b1
b1
A:b2
b2
=====================

Actual behavior

A:b1
b1
A:b2
b2
=====================

A:b1
b1
b1
=====================

Environment

• Version: 6.9.1

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[thisisthekap]

@glennawatson As it looks like this issue was somehow forgotten, this is just a friendly ping :)

[glennawatson]

@thisisthekap -- I don't usually maintain Akavache, sort of maintain most of the RxUI projects. The most recent maintainer works at Microsoft and a lot of his time has definitely been sunk into working there, so I'm just slowly getting to Akavache issues as I can. This seems to a good one to fix up so I will look at it some point in the next couple weeks.