public void ShouldThrowExceptionWhenKeyIsNullOrEmpty(string key)
{
Assert.Throws <ArgumentNullException>(() => _cache.Contains(key));
Assert.Throws <ArgumentNullException>(() => _cache.Get(key));
Assert.Throws <ArgumentNullException>(() => _cache.Set(key, "value"));
Assert.Throws <ArgumentNullException>(() => _cache.Remove(key));
}
